[libcxxabi] Build all of libcxxabi with _LIBCPP_BUILDING_LIBRARY defined

[mstorsjo]

Summary

Various definitions from libcxx need to be set in the same way as if building libcxx itself.

Split out from D90021 as requested by @ldionne.

The comment/define in fallback_malloc.cpp seems to be a bit stale; it was added in 4b47cbccd453c089f0a1d1699dd80718943986d0, but lost its current effect in 42f9868cd80d2e09c75f4642e0a66c7ecd38cf18 and the comment was reworded in 51fbb2e70ae8a2a8df24845f7b6fdf4a1aebce4f without touching the define. - @EricWF

This change in itself shouldn't have any effect on that aspect in fallback_malloc.cpp (whether it works as intended or not) as we still define _LIBCPP_BUILDING_LIBRARY, just defined projectwide now.

[compnerd]

This is definitely a good cleanup IMO. I wonder if this should be taken a step further and use the "standard" (well, as far as CMake is concerned) standard macros of LIBCXX_EXPORTS. Of course, that is probably done better in a subsequent change.

[smeenai]

In D90476#2367494, @compnerd wrote:

This is definitely a good cleanup IMO. I wonder if this should be taken a step further and use the "standard" (well, as far as CMake is concerned) standard macros of LIBCXX_EXPORTS. Of course, that is probably done better in a subsequent change.

IIRC, we used to rely on this at some point, and then libc++ changed to use a common object library for both the static and shared libraries, at which point you couldn't rely on CMake to define that. I think we we build the static and shared libraries as separate libraries again rather than using the object library (to get PIC right), so the default export macro should be available to us again.

libcxxabi/CMakeLists.txt

if (LIBCXXABI_HAS_NOSTDINCXX_FLAG)
   string(REPLACE "-stdlib=libc++" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
   string(REPLACE "-stdlib=libstdc++" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
 endif()
 
 # Let the library headers know they are currently being used to build the
 # library.
 add_definitions(-D_LIBCXXABI_BUILDING_LIBRARY)
 
+# libcxxabi needs to, for various reasons, include the libcpp headers as if
+# it is being built as part of libcxx.
+add_definitions(-D_LIBCPP_BUILDING_LIBRARY)
+
 # Disable DLL annotations on Windows for static builds.
 if (WIN32 AND LIBCXXABI_ENABLE_STATIC AND NOT LIBCXXABI_ENABLE_SHARED)
   add_definitions(-D_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS)
 endif()
 
 add_compile_flags_if_supported(-Werror=return-type)
 
 # Get warning flags

libcxxabi/src/fallback_malloc.cpp

 //===------------------------ fallback_malloc.cpp -------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
-// Define _LIBCPP_BUILDING_LIBRARY to ensure _LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION
-// is only defined when libc aligned allocation is not available.
-#define _LIBCPP_BUILDING_LIBRARY
 #include "fallback_malloc.h"
 
 #include <__threading_support>
 #ifndef _LIBCXXABI_HAS_NO_THREADS
 #if defined(__ELF__) && defined(_LIBCXXABI_LINK_PTHREAD_LIB)
 #pragma comment(lib, "pthread")
 #endif
 #endif

libcxxabi/src/stdlib_exception.cpp

 //===---------------------------- exception.cpp ---------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
-#define _LIBCPP_BUILDING_LIBRARY
 #include <new>
 #include <exception>
 
 namespace std
 {
 
 // exception
 

libcxxabi/src/stdlib_new_delete.cpp

 //===--------------------- stdlib_new_delete.cpp --------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //
 // This file implements the new and delete operators.
 //===----------------------------------------------------------------------===//
 
-#define _LIBCPP_BUILDING_LIBRARY
 #include "__cxxabi_config.h"
 #include <new>
 #include <cstdlib>
 
 #if !defined(_THROW_BAD_ALLOC) || !defined(_NOEXCEPT) || !defined(_LIBCXXABI_WEAK)
 #error The _THROW_BAD_ALLOC, _NOEXCEPT, and _LIBCXXABI_WEAK libc++ macros must \
        already be defined by libc++.
 #endif