Add default C++ ABI libname and include paths for FreeBSD

Summary:
As noted in a discussion about testing the LLVM 6.0.0 release candidates
(with libc++) for FreeBSD, many tests turned out to fail with
"exception_ptr not yet implemented".  This was because libc++ did not
choose the correct C++ ABI library, and therefore it fell back to the
`exception_fallback.ipp` header.

Since FreeBSD 10.x, we have been using libcxxrt as our C++ ABI library,
and its headers have always been installed in /usr/include/c++/v1,
together with the (system) libc++ headers.  (Older versions of FreeBSD
used GNU libsupc++ by default, but these are now unsupported.)

Therefore, if we are building libc++ for FreeBSD, set:
* `LIBCXX_CXX_ABI_LIBNAME` to "libcxxrt"
* `LIBCXX_CXX_ABI_INCLUDE_PATHS` to "/usr/include/c++/v1"
by default.

Reviewers: emaste, EricWF, mclow.lists

Reviewed By: EricWF

Subscribers: mgorny, cfe-commits, krytarowski

Differential Revision: https://reviews.llvm.org/D43166

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@324855 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed