Bug 1426283 - Work around bug 1409276. r=gps

Both the cc crate and the rust compiler may want to use "cc", which,
on automation, points to the system GCC compiler instead of ours.

As a workaround, we add a cc symbolic link in the GCC toolchain artifact
so that, as long as the GCC toolchain artifact's bin directory is in
$PATH early enough, it's picked over /usr/bin/cc.

--HG--
extra : rebase_source : 53cacf8a750539706a484218e168c8c9e0ba49a6

build/unix/build-gcc/build-gcc.sh

@@ -116,6 +116,7 @@ build_gcc() {
   make $make_flags install DESTDIR=$root_dir
 
   cd $root_dir/tools
+  ln -s gcc gcc/bin/cc
   tar caf $root_dir/gcc.tar.xz gcc/
   popd
 }