do not install all triplets

Hi,

I noticed all the triplets were installed, this fixes it, it only installs the symlinks <mingw-triplet>-clang[++] for which a <mingw-triplet>-gcc were found.

I did not know to write this cleanly with autotools, and I'm much conmfortable with cmake, as it remove the need for shell/m4/autoconf scripts scattered around, all the infrastructure is written in neat cmakelists.txt.

For discussion.