depends: fix BDB compilation on OpenBSD

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. 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.

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.

Tested on OpenBSD 7.4 with clang 13.0.0.

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

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