[OMPT] Fix failing test cases for gcc on Ubuntu

The compiler warns that _BSD_SOURCE is deprecated and _DEFAULT_SOURCE should
be used instead. We keep _BSD_SOURCE for older compilers, that don't know
about _DEFAULT_SOURCE.

The linker drops the tool when linking, since there is no visible need for
the library. So we need to tell the linker, that the tool should be linked
anyway.

Differential Revision: https://reviews.llvm.org/D41499

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@321362 91177308-0d34-0410-b5e6-96231b3b80d8