[CMake][libcxx] Check that we have libcxxabi before using it

[phosek]

Summary

When doing standalone build, check that we actually have libcxxabi before attempting to use it.

[krasin]

CMakeLists.txt

122

This condition is hard to read. What's about

if (HAVE_LIBCXXABI OR NOT LIBCXX_STANDALONE_BUILD) AND
    IS_DIRECTORY "${LIBCXX_LIBCXXABI_INCLUDES_INTERNAL}")

Also, can you please elaborate the parts of this condition a bit? I don't follow the logic. How do HAVE_LIBCXXABI, LIBCXX_STANDALONE_BUILD and LIBCXX_LIBCXXABI_INCLUDES_INTERNAL play with each other?

[krasin]

LGTM

[phosek]

We now have basically 3 modes of operations when building libc++:

1. building libc++ as part of LLVM using projects/ when neither LIBCXX_STANDALONE_BUILD is nor HAVE_LIBCXXABI is defined, but we to link against libc++abi if it's available hence the IS_DIRECTORY check
2. building libc++ as part of LLVM using runtimes/ when LIBCXX_STANDALONE_BUILD is set and HAVE_LIBCXXABI will be set as well if libc++abi is being built as part of runtimes/ in which case we want to link against it
3. building libc++ standalone when only LIBCXX_STANDALONE_BUILD is set in which case we shouldn't be using libc++abi even if it's present (the IS_DIRECTORY is true) because it might not be built

With D28579, we'll be building libc++ for TSan as standalone, but since we aren't building libc++abi as well we shouldn't attempt to use even if it's in the tree hence the change in the condition.

CMakeLists.txt

 if (LIBCXX_CXX_ABI STREQUAL "default")
   find_path(
     LIBCXX_LIBCXXABI_INCLUDES_INTERNAL
     cxxabi.h
     PATHS ${LLVM_MAIN_SRC_DIR}/projects/libcxxabi/include
           ${LLVM_MAIN_SRC_DIR}/runtimes/libcxxabi/include
     NO_DEFAULT_PATH
   )
-  if (IS_DIRECTORY "${LIBCXX_LIBCXXABI_INCLUDES_INTERNAL}")
+  if (NOT (LIBCXX_STANDALONE_BUILD AND NOT 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)
   else()
     if (WIN32 AND NOT MINGW)
       # FIXME: Figure out how to configure the ABI library on Windows.
       set(LIBCXX_CXX_ABI_LIBNAME "none")
     else()