[openjdk / icedtea patch] allow to build with versioned compiler

[Thomas Fitzsimmons]

Matthias Klose wrote:
> its somewhat difficult to set the compiler to something different than gcc/g++.
> Adding another bin dir to PATH doesn't work because the compiler is called with
> an absolute path (why)? Adding a GCC_VERSION macro (which can be set in the
> environment), works for me to build using a specific compiler version.

This looks fine; can you test it for b20, add the Makefile.am bits and resubmit?

Tom