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

llvm-svn: 324855

libcxx/CMakeLists.txt

@@ -136,6 +136,9 @@ if (LIBCXX_CXX_ABI STREQUAL "default")
   elseif (APPLE)
     set(LIBCXX_CXX_ABI_LIBNAME "libcxxabi")
     set(LIBCXX_CXX_ABI_SYSTEM 1)
+  elseif (${CMAKE_SYSTEM_NAME} MATCHES "FreeBSD")
+    set(LIBCXX_CXX_ABI_LIBNAME "libcxxrt")
+    set(LIBCXX_CXX_ABI_INCLUDE_PATHS "/usr/include/c++/v1")
   else()
     set(LIBCXX_CXX_ABI_LIBNAME "default")
   endif()