[noupstream] Add GMP and MPFR to our GCC repository.

This would allow us to remove the respective tarballs from SVN.

As before, we would use this build time optimization: if the correct versions of
packages are installed in the system, we would use those. This optimization is
most important on Windows/Cygwin where build time is significantly bigger than
on Linux/Mac.

To let the GCC build take the sources of GMP and MPFR one would need to create
ln -s gmp-4.3.1 gmp
ln -s mpfr-2.4.1 mpfr

GMP: 4.3.1
MPFR: 2.4.1

The files were obtained by unpacking the tarballs in third_party/gcc at r4214.

Rietveld was unable to accept the upload of whole set of files, HTTP/500, you
know, so committing without review.
1764 files changed