Merge bitcoin/bitcoin#32828: build, docs: Fix Boost-related issues on NetBSD

5a5ddbd789 build: Add workaround for NetBSD bug in `Boost::headers` target (Hennadii Stepanov)
4eb3cee919 doc: Update NetBSD Build Guide (Hennadii Stepanov)

Pull request description:

  The recently merged https://github.com/bitcoin/bitcoin/pull/32667 broke builds on NetBSD due to the following issues:

  1. The `boost-headers` package does not provide CMake configuration files now required since https://github.com/bitcoin/bitcoin/pull/32667.

     The first commit updates the Build Guide to recommend using the `boost` package instead.

  2. The CMake configuration file provided by the `boost` package is [broken](https://github.com/NetBSD/pkgsrc/issues/167).

     The second commit adds a workaround for this issue.

  Upstream issues:
  - https://github.com/NetBSD/pkgsrc/issues/167
  - https://github.com/NetBSD/pkgsrc/issues/168

  Successful CI job: https://github.com/hebasto/bitcoin-core-nightly/actions/runs/15958818914/job/45008576879.

ACKs for top commit:
  purpleKarrot:
    ACK 5a5ddbd789

Tree-SHA512: 617543edf2216c3673195cc562541d7637d7ea199320916c3660a91f6ee1ff01c014f519c5eeb3fc224f7a7304fdb75965ee3f4554d7fb277dd05ba3a6583b45

cmake/module/AddBoostIfNeeded.cmake

@@ -31,6 +31,15 @@ function(add_boost_if_needed)
 
   find_package(Boost 1.73.0 REQUIRED CONFIG)
   mark_as_advanced(Boost_INCLUDE_DIR boost_headers_DIR)
+  # Workaround for a bug in NetBSD pkgsrc.
+  # See: https://github.com/NetBSD/pkgsrc/issues/167.
+  if(CMAKE_SYSTEM_NAME STREQUAL "NetBSD")
+    get_filename_component(_boost_include_dir "${boost_headers_DIR}/../../../include/" ABSOLUTE)
+    set_target_properties(Boost::headers PROPERTIES
+      INTERFACE_INCLUDE_DIRECTORIES ${_boost_include_dir}
+    )
+    unset(_boost_include_dir)
+  endif()
   set_target_properties(Boost::headers PROPERTIES IMPORTED_GLOBAL TRUE)
   target_compile_definitions(Boost::headers INTERFACE
     # We don't use multi_index serialization.