[PATCH] Fix gcc-5-branch build with libc++

[David Edelsohn]

> As discussed with the FreeBSD gcc ports maintainer, building the
> gcc-5-branch with libc++ requires standard C++ headers to be included
> *before* gcc/system.h, otherwise the redefinition of abort() to
> fancy_abort() will cause trouble.
>
> In trunk r235362, most gcc C++ sources were updated to define
> INCLUDE_xxx macros before including gcc/system.h, which fixes the
> incompatibility with libc++.  However, this revision is most likely too
> disruptive to backport to the gcc-5-branch.

As discussed in the thread to define INCLUDE_xxx macros, no headers
may be included before GCC system.h because it breaks the GCC build on
some systems.  The inclusion of C++ headers prior to system.h pulls in
system headers that define global macros in conflict with system.h and
system headers included later in the sequence.

The developers who slipped in C++ headers did so in error, and it has
been corrected.

Anyone who suggested to the FreeBSD community that it would be
acceptable to include C++ headers early was severely mistaken.  Sorry.

Thanks, David