build: wscript. Fix the build on FreeBSD with the clang ld.lld-XX linker.

build: wscript. Fix the build on FreeBSD with the clang ld.lld-XX linker.

FreeBSD is broken. It doesn't include 'extern char **environ'
in any shared library, but statically inside crt0.o.

If we're running on a FreeBSD with the GNU linker ld we
can get around this by explicitly telling the linker to
ignore 'environ' as an unresolved symbol in a shared library.

However, the clang linker ld.lld-XX is broken in that it
doesn't have that option.
First try to see if have '-Wl,--ignore-unresolved-symbol,environ'
and just use that if so.

If not, we have to use '-Wl,--allow-shlib-undefined' instead
and remove all instances of '-Wl,-no-undefined'.

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Martin Schwenke <martin@meltin.net>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Fri May 14 01:50:25 UTC 2021 on sn-devel-184