Added option to build with gcc and/or clang:

--compiler-vendors=<gcc,clang>

.. you need the compilers for every system that you
build for unfortunately; but then again, having the
complete set of compilers for every system is quite
a good idea, and this project is used as a test-bed
for improving and adding Darwin / Clang / MinGW-w64
support to crosstool-ng here:

https://github.com/diorcety/crosstool-ng