Fix build with GCC with recent head kernel by disabling PCH for GCC.

The case for this is a recent head kernel building an older branch that uses
GCC in a jail.

This is discussed at https://lists.freebsd.org/pipermail/svn-src-all/2015-March/101722.html

It may be possible to fix GCC to do the right thing with mmap(2) but it would
not be simple to make ports use a fixed GCC on older releases and without
needlessly building a ports compiler when the system one would otherwise be
fine without PCH.

With hat: portmgr