Each recipe consists of 3 main parts: defining identifiers, setting build
variables, and defining build commands.

The package "mylib" will be used here as an example

General tips:
- mylib_foo is written as $(package)_foo in order to make recipes more similar.
- Secondary dependency packages relative to the bitcoin binaries/libraries (i.e.
  those not in `ALLOWED_LIBRARIES` in `contrib/devtools/symbol-check.py`) don't
  need to be shared and should be built statically whenever possible. See
  [below](#secondary-dependencies) for more details.

## Identifiers
Each package is required to define at least these variables:

    $(package)_version:
    Version of the upstream library or program. If there is no version, a
    placeholder such as 1.0 can be used.

    $(package)_download_path:
    Location of the upstream source, without the file-name. Usually http, https
    or ftp. Secure transmission options like https should be preferred if
    available.

    $(package)_file_name:
    The upstream source filename available at the download path.

    $(package)_sha256_hash:
    The sha256 hash of the upstream file

These variables are optional:

    $(package)_build_subdir:
    cd to this dir before running configure/build/stage commands.

    $(package)_download_file:
    The file-name of the upstream source if it differs from how it should be
    stored locally. This can be used to avoid storing file-names with strange
    characters.

    $(package)_dependencies:
    Names of any other packages that this one depends on.

    $(package)_patches:
    Filenames of any patches needed to build the package

    $(package)_extra_sources:
    Any extra files that will be fetched via $(package)_fetch_cmds. These are
    specified so that they can be fetched and verified via 'make download'.


## Build Variables:
After defining the main identifiers, build variables may be added or customized
before running the build commands. They should be added to a function called
$(package)_set_vars. For example:

    define $(package)_set_vars
    ...
    endef

Most variables can be prefixed with the host, architecture, or both, to make
the modifications specific to that case. For example:

    Universal:     $(package)_cc=gcc
    Linux only:    $(package)_linux_cc=gcc
    x86_64 only:       $(package)_x86_64_cc = gcc
    x86_64 linux only: $(package)_x86_64_linux_cc = gcc

These variables may be set to override or append their default values.

    $(package)_cc
    $(package)_cxx
    $(package)_objc
    $(package)_objcxx
    $(package)_ar
    $(package)_ranlib
    $(package)_libtool
    $(package)_nm
    $(package)_cflags
    $(package)_cxxflags
    $(package)_ldflags
    $(package)_cppflags
    $(package)_config_env
    $(package)_build_env
    $(package)_stage_env
    $(package)_build_opts
    $(package)_config_opts

The *_env variables are used to add environment variables to the respective
commands.

Many variables respect a debug/release suffix as well, in order to use them for
only the appropriate build config. For example:

    $(package)_cflags_release = -O3
    $(package)_cflags_i686_debug = -g
    $(package)_config_opts_release = --disable-debug

These will be used in addition to the options that do not specify
debug/release. All builds are considered to be release unless DEBUG=1 is set by
the user. Other variables may be defined as needed.

## Build commands:

  For each build, a unique build dir and staging dir are created. For example,
  `work/build/mylib/1.0-1adac830f6e` and `work/staging/mylib/1.0-1adac830f6e`.

  The following build commands are available for each recipe:

    $(package)_fetch_cmds:
    Runs from: build dir
    Fetch the source file. If undefined, it will be fetched and verified
    against its hash.

    $(package)_extract_cmds:
    Runs from: build dir
    Verify the source file against its hash and extract it. If undefined, the
    source is assumed to be a tarball.

    $(package)_preprocess_cmds:
    Runs from: build dir/$(package)_build_subdir
    Preprocess the source as necessary. If undefined, does nothing.

    $(package)_config_cmds:
    Runs from: build dir/$(package)_build_subdir
    Configure the source. If undefined, does nothing.

    $(package)_build_cmds:
    Runs from: build dir/$(package)_build_subdir
    Build the source. If undefined, does nothing.

    $(package)_stage_cmds:
    Runs from: build dir/$(package)_build_subdir
    Stage the build results. If undefined, does nothing.

  The following variables are available for each recipe:

    $(1)_staging_dir: package's destination sysroot path
    $(1)_staging_prefix_dir: prefix path inside of the package's staging dir
    $(1)_extract_dir: path to the package's extracted sources
    $(1)_build_dir: path where configure/build/stage commands will be run
    $(1)_patch_dir: path where the package's patches (if any) are found

Notes on build commands:

For packages built with autotools, $($(package)_autoconf) can be used in the
configure step to (usually) correctly configure automatically. Any
$($(package)_config_opts) will be appended.

Most autotools projects can be properly staged using:

    $(MAKE) DESTDIR=$($(package)_staging_dir) install

## Build outputs:

In general, the output of a depends package should not contain any libtool
archives. Instead, the package should output `.pc` (`pkg-config`) files where
possible.

From the [Gentoo Wiki entry](https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Handling_Libtool_Archives):

>  Libtool pulls in all direct and indirect dependencies into the .la files it
>  creates. This leads to massive overlinking, which is toxic to the Gentoo
>  ecosystem, as it leads to a massive number of unnecessary rebuilds.

## Secondary dependencies:

Secondary dependency packages relative to the bitcoin binaries/libraries (i.e.
those not in `ALLOWED_LIBRARIES` in `contrib/devtools/symbol-check.py`) don't
need to be shared and should be built statically whenever possible. This
improves general build reliability as illustrated by the following example:

When linking an executable against a shared library `libprimary` that has its
own shared dependency `libsecondary`, we may need to specify the path to
`libsecondary` on the link command using the `-rpath/-rpath-link` options, it is
not sufficient to just say `libprimary`.

For us, it's much easier to just link a static `libsecondary` into a shared
`libprimary`. Especially because in our case, we are linking against a dummy
`libprimary` anyway that we'll throw away. We don't care if the end-user has a
static or dynamic `libsecondary`, that's not our concern. With a static
`libsecondary`, when we need to link `libprimary` into our executable, there's no
dependency chain to worry about as `libprimary` has all the symbols.

## Build targets:

To build an individual package (useful for debugging), following build targets are available.

    make ${package}
    make ${package}_fetched
    make ${package}_extracted
    make ${package}_preprocessed
    make ${package}_configured
    make ${package}_built
    make ${package}_staged
    make ${package}_postprocessed
    make ${package}_cached
    make ${package}_cached_checksum
