devel/gprbuild: Upgrade version 2014 => 2015

This upgrade was necessary because the latest version introduces new
switches that Adacore software already uses. I'd been working around
it be replacing, for example, "gprinstall" with manual install targets.

More importantly, gprbuild is built with project files. This requires
either gnatmake (which will drop this functionality on next gcc version)
or a copy of gprbuild already installed. Since I don't want to carry
binary bootstraps, I wrote a new BSD-style makefile to avoid gprbuild
as a dependency for itself.

Unlike GPS 2014, it uses the gnat sources that it was provided with
instead of gnat_util library. The gpr* sources use functions not yes
in FSF GNAT and only one regression patch is needed when built by
gcc5-aux. This probably cannot be built by gcc-aux, but there are
already several things preventing the return to gcc-aux (over gcc5-aux).

I didn't bother with the examples so these are no longer installed, and
I didn't install "info" version of documention which follows my other
ports. (pdf, txt, and html versions are still installed)