Switch stdenv to GCC 4.8.0.
gmp-4.3.2.nix: disable test suite to fix the build

The test t-lucnum_ui fails (on Linux/x86_64) when built with GCC 4.8.
Newer versions of GMP don't have that issue anymore.
Merge pull request #445 from peti/switch-stdenv-to-gcc-4.8.0

Switch stdenv to GCC 4.8.0.