Similar to bug 212465, lang/gcc5 doesn't compile with recent versions of
libc++, because it attempts to redefine abort():

In file included from /wrkdirs/usr/ports/lang/gcc5/work/gcc-5.4.0/gcc/auto-profile.c:25:
In file included from /usr/include/c++/v1/map:446:
/usr/include/c++/v1/functional:1398:2: error: no member named 'fancy_abort' in namespace 'std::__1'; did you mean simply 'fancy_abort'?

_VSTD::abort();
^~~~~~~

/usr/include/c++/v1/__config:383:15: note: expanded from macro '_VSTD'
#define _VSTD std::_LIBCPP_NAMESPACE

^

Patch this in the same way as the other gcc ports, by including <new> in
gcc/system.h, and moving a few includes to before "system.h".

Approved by: gerald (maintainer)
PR: 216266
MFH: 2017Q1