Allow building without threads in standalone builds

[matthewbauer]

Summary

Setting _LIBCPP_HAS_NO_THREADS is needed when building libcxxabi without threads in standalone mode. This is useful when target WASM. Otherwise, you get an error like "No thread API" when building libcxxabi.

[ldionne]

Can you please add context to the diff?

[matthewbauer]

Added more context for CMakeLists.txt

[mclow.lists]

Why is this necessary? I just grepped the libc++abi sources, and I see no only two references to _LIBCPP_HAS_NO_THREADS, and they're both in comments (which should be fixed, btw, but that's neither here nor there).

[matthewbauer]

In D60743#1469145, @mclow.lists wrote:

Why is this necessary? I just grepped the libc++abi sources, and I see no only two references to _LIBCPP_HAS_NO_THREADS, and they're both in comments (which should be fixed, btw, but that's neither here nor there).

libcxx headers are included by libcxxabi via LIBCXXABI_LIBCXX_INCLUDES. For instance "src/include/atomic_support.h" includes "__config" header here:

https://github.com/llvm-mirror/libcxxabi/blob/master/src/include/atomic_support.h#L16

[ldionne]

The fact that we have to add this manually is unfortunate.

Better solution: define a CMake target representing the libc++ headers which has the right defines as part of its INTERFACE, and then have libc++abi link to these headers. Then have cxx_shared/cxx_static link to the headers AND to cxxabi_<shared|static>. Will refactor eventually, one thing at a time.

libcxxabi/CMakeLists.txt

message(FATAL_ERROR "LIBCXXABI_HAS_EXTERNAL_THREAD_API can only"
                         " is also set to ON.")
   endif()
   if (LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY)
     message(FATAL_ERROR "LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY can only"
                         " be set to ON when LIBCXXABI_ENABLE_THREADS"
                         " is also set to ON.")
   endif()
   add_definitions(-D_LIBCXXABI_HAS_NO_THREADS)
+  add_definitions(-D_LIBCPP_HAS_NO_THREADS)
 endif()
 
 if (LIBCXXABI_HAS_EXTERNAL_THREAD_API)
   if (LIBCXXABI_HAS_PTHREAD_API)
     message(FATAL_ERROR "The options LIBCXXABI_HAS_EXTERNAL_THREAD_API"
                         " and LIBCXXABI_HAS_PTHREAD_API cannot be both"
                         " set to ON at the same time.")
   endif()