Add a new LIBUNWIND_WEAK_PTHREAD Cmake option to force
calls into the pthread library use weak symbols.

This option allows libpthread to be a weak dependency rather
than a hard one.

Differential Revision: https://reviews.llvm.org/D60285

llvm-svn: 360610
GitOrigin-RevId: 7981a28d9d303e9e209c16cb2749bc6f47a5b521
3 files changed