re PR other/4372 (#pragma weak pthread* inclusion causes applications to crash without a linker error when one forgets to link with -lpthread)

gcc/ChangeLog:
PR other/4372
* gthr-dce.h, gthr-posix.h, gthr-posix95.h, gthr-solaris.h,
gthr-tpf.h: Define __gthrw.  For all identifiers that might
be weak, introduce weakrefs or non-weak aliases with __gthrw,
and prefix all uses with __ghtrw.
libobjc/ChangeLog:
PR other/4372
* thr-objc.c (_XOPEN_SOURCE): Define.

From-SVN: r106704