Fix build of lang/gcc49 with libc++ 3.9.0

While testing the clang390-import branch, I ran into the following
errors building lang/gcc49:

In file included from /wrkdirs/usr/ports/lang/gcc49/work/gcc-4.9.4/gcc/c/c-objc-common.c:33:
In file included from /usr/include/c++/v1/new:70:
/usr/include/c++/v1/exception:267:5: error: no member named 'fancy_abort' in namespace 'std::__1'; did you mean simply 'fancy_abort'?

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

/usr/include/c++/v1/__config:451:15: note: expanded from macro '_VSTD'
#define _VSTD std::_LIBCPP_NAMESPACE

^

/wrkdirs/usr/ports/lang/gcc49/work/gcc-4.9.4/gcc/system.h:685:13: note: 'fancy_abort' declared here
extern void fancy_abort (const char *, int, const char *) ATTRIBUTE_NORETURN;

^

1 error generated.

What is happening here, is that the source file includes gcc/system.h,
which defines abort to fancy_abort, and then the source file includes
<new>, which attempts to call _VSTD::abort() (the _VSTD is a libc++
alias for std::). The macro definition then causes the above breakage.

Newer gcc ports, such as gcc5 and gcc6 don't show this issue, because
upstream gcc first added an include of <algorithm> (which indirectly
includes <new>) in r217348 [1], and later even add a direct include of
<new> in r232736 [2].

Fix it for this version, by adding the direct include of <new> to
gcc/system.h. This makes the 'second' includes of <new> in some .c
files superfluous, but at least they won't result in errors.

[1] https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=217348
[2] https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=232736

Approved by: gerald (maintainer)
PR: 212465
MFH: 2016Q3