Saleem Abdulrasool: Ensure that __GLIBCXX__ is defined when building with libsupc++.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@173164 91177308-0d34-0410-b5e6-96231b3b80d8
Saleem Abdulrasool: __terminate_handler and __unexpected_handler are defined but not used when
building against libsupc++ as the functions for which they are used are provided
by libsupc++.  Simply preprocess them away when building against libsupc++.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@173165 91177308-0d34-0410-b5e6-96231b3b80d8