Merge bitcoin/bitcoin#29443: depends: fix BDB compilation on OpenBSD

0fbf051fec depends: fix BDB compilation on OpenBSD (Sebastian Falbesoner)

Pull request description:

  Compiling C++ code with `-D_XOPEN_SOURCE=600` causes problems on OpenBSD. If that define is set, the C++ standard header detection routine in BDB's configure script fails due to a missing type name for `locale_t` (see https://gist.github.com/theStack/b41884e31ebc5cdca3220bcaa674cb70 for the relevant config.log part).

  This results in `HAVE_CXX_STDHEADERS` not being defined, which then it turn leads to the inclusion of `<iostream.h>` (rather than `<iostream>`), which doesn't exist, as described in #28963.

  According to a mailing list post discussing a similar problem [1], "OpenBSD provides the POSIX APIs by default", so we don't need this define anyway and can remove it. This fixes the BDB build problem as described in issue #28963. See also f87e75ae71 for a similar fix for google's flatbuffer project.

  Tested on OpenBSD 7.4 with clang 13.0.0. Fixes #28963.

  [1] https://www.mail-archive.com/tech@openbsd.org/msg63386.html

ACKs for top commit:
  fanquake:
    ACK 0fbf051fec

Tree-SHA512: 02139e9081ed855e067bfba8c81b54c657417576e553cc1035a916ada9be049358f5e14d756d5f234c5226bd7e943f61c6ae8990c1b152f9125681b7b777c9b3

depends/packages/bdb.mk

@@ -17,7 +17,6 @@ $(package)_config_opts_android=--with-pic
 $(package)_cflags+=-Wno-error=implicit-function-declaration -Wno-error=format-security -Wno-error=implicit-int
 $(package)_cppflags_freebsd=-D_XOPEN_SOURCE=600 -D__BSD_VISIBLE=1
 $(package)_cppflags_netbsd=-D_XOPEN_SOURCE=600
-$(package)_cppflags_openbsd=-D_XOPEN_SOURCE=600
 $(package)_cppflags_mingw32=-DUNICODE -D_UNICODE
 endef