[libc++] Don't override LIBCXX_CXX_ABI_INCLUDE_PATHS for libcxxrt

[arichardson]

Summary

When testing libc++ for our cross-compiled CheriBSD target we specify an
explicit LIBCXX_CXX_ABI_INCLUDE_PATHS for libcxxrt. The hardcoded path
/usr/include/c++/v1 was introduced in 61e89737c5daf7b65341936f88f68efb680cdcd4
and overrides any value passed on the CMake command line. Fix this by using
it as a fallback rather than a fixed default value.

[ldionne]

Can you also please make the change for the elseif ("${LIBCXX_CXX_ABI_LIBNAME}" STREQUAL "libcxxabi") case?

[arichardson]

Handle the other branch

libcxx/cmake/Modules/HandleLibCXXABI.cmake

else()
     set(_LIBSUPCXX_DEFINES "")
     set(_LIBSUPCXX_LIBNAME supc++)
   endif()
   setup_abi_lib(
     "-D__GLIBCXX__ ${_LIBSUPCXX_DEFINES}"
     "${_LIBSUPCXX_LIBNAME}" "${_LIBSUPCXX_LIBNAME}" "${_LIBSUPCXX_INCLUDE_FILES}" "bits"
     )
 elseif ("${LIBCXX_CXX_ABI_LIBNAME}" STREQUAL "libcxxabi")
+  if(NOT LIBCXX_CXX_ABI_INCLUDE_PATHS)
     set(LIBCXX_CXX_ABI_INCLUDE_PATHS "${LIBCXX_SOURCE_DIR}/../libcxxabi/include")
+  endif()
 
   if(LIBCXX_STANDALONE_BUILD AND NOT (LIBCXX_CXX_ABI_INTREE OR HAVE_LIBCXXABI))
     set(shared c++abi)
     set(static c++abi)
   else()
     set(shared cxxabi_shared)
     set(static cxxabi_static)
   endif()
 
   setup_abi_lib(
     "-DLIBCXX_BUILDING_LIBCXXABI"
     "${shared}" "${static}" "cxxabi.h;__cxxabi_config.h" "")
 elseif ("${LIBCXX_CXX_ABI_LIBNAME}" STREQUAL "libcxxrt")
+  if(NOT LIBCXX_CXX_ABI_INCLUDE_PATHS)
     set(LIBCXX_CXX_ABI_INCLUDE_PATHS "/usr/include/c++/v1")
+  endif()
   setup_abi_lib(
     "-DLIBCXXRT"
     "cxxrt" "cxxrt" "cxxabi.h;unwind.h;unwind-arm.h;unwind-itanium.h" ""
     )
 elseif ("${LIBCXX_CXX_ABI_LIBNAME}" STREQUAL "vcruntime")
  # Nothing to do
 elseif ("${LIBCXX_CXX_ABI_LIBNAME}" STREQUAL "none")
   list(APPEND LIBCXX_COMPILE_FLAGS "-D_LIBCPP_BUILDING_HAS_NO_ABI_LIBRARY")