[libc++abi] fix compilation in C++17 mode

[rsmith]

Summary

C++17 removes std::unexpected_handler, but libc++abi needs it to define __cxa_exception. When building against libc++, this is easily rectified by telling libc++ we're building the library. We already do this in the other places where we need these symbols.

[mclow.lists]

I'm fine with this, though I wonder if using the more specific _LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS might be a better thing to define.

[EricWF]

I agree with Marshall. This should define _LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS instead of _LIBCPP_BUILDING_LIBRARY. Other than that, this LGTM.

[rsmith]

I defined _LIBCPP_BUILDING_LIBRARY instead to match the existing code in libc++abi (stdlib_exception.cpp, stdlib_new_delete.cpp), which uses the _BUILDING_LIBRARY macro as a general way to say "give me all the symbols because I'm building the library". That seemed to match the semantics of this code better than a macro whose purpose is "I am writing user code that is still using this removed symbol".

But I don't really care. If you two prefer the other macro, that's fine with me :)

(Would there be any interest in a patch that just unconditionally defines _LIBCPP_BUILDING_LIBRARY throughout the libc++abi compilation instead of this change?)

[EricWF]

@rsmith. Sorry, you're right. I didn't notice that we already used _LIBCPP_BUILDING_LIBRARY in libc++abi. I wasn't sure if _BUILDING_LIBRARY changed the linkage or of any symbols, or changed their explicit instantiation.
And, indeed, on Windows there's a problem with DLL import/export macros. I'll fix and cleanup the usage of _BUILDING_LIBRARY in the next couple days.

libcxxabi/trunk/src/cxa_exception.cpp

 //===------------------------- cxa_exception.cpp --------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is dual licensed under the MIT and the University of Illinois Open
 // Source Licenses. See LICENSE.TXT for details.
 //
 //  
 //  This file implements the "Exception Handling APIs"
 //  http://mentorembedded.github.io/cxx-abi/abi-eh.html
 //  
 //===----------------------------------------------------------------------===//
 
+#define _LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS
+
 #include "cxxabi.h"
 
 #include <exception>        // for std::terminate
 #include <cstring>          // for memset
 #include "cxa_exception.hpp"
 #include "cxa_handlers.hpp"
 #include "fallback_malloc.h"