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

[dim]

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.

[EricWF]

CMakeLists.txt

141

Actually, I'm not sure how much I love having other libc++ header on the include path... I'm not sure how well this works especially with out forwarding header. Maybe what would be appropriate would be to copy the cxxabi.h header into the build directory?

[EricWF]

CMakeLists.txt

141

Nevermind... That's what this already does. My bad.

CMakeLists.txt

if (LIBCXX_CXX_ABI STREQUAL "default")
   elseif ((NOT LIBCXX_STANDALONE_BUILD OR HAVE_LIBCXXABI) AND
           IS_DIRECTORY "${LIBCXX_LIBCXXABI_INCLUDES_INTERNAL}")
     set(LIBCXX_CXX_ABI_LIBNAME "libcxxabi")
     set(LIBCXX_CXX_ABI_INCLUDE_PATHS "${LIBCXX_LIBCXXABI_INCLUDES_INTERNAL}")
     set(LIBCXX_CXX_ABI_INTREE 1)
   elseif (APPLE)
     set(LIBCXX_CXX_ABI_LIBNAME "libcxxabi")
     set(LIBCXX_CXX_ABI_SYSTEM 1)
+  elseif (${CMAKE_SYSTEM_NAME} MATCHES "FreeBSD")
+    set(LIBCXX_CXX_ABI_LIBNAME "libcxxrt")
+    set(LIBCXX_CXX_ABI_INCLUDE_PATHS "/usr/include/c++/v1")
   else()
     set(LIBCXX_CXX_ABI_LIBNAME "default")
   endif()
 else()
   set(LIBCXX_CXX_ABI_LIBNAME "${LIBCXX_CXX_ABI}")
 endif()
 
 # Use a static copy of the ABI library when linking libc++. This option