Gmp4: Fix legacy (not working) compiler check.
Gmp4: Fix legacy (not working) compiler check.

* Keg_only because of conflict with gmp.

UniqMartin/homebrew-versions,jimhe/homebrew-versions,heuermh/homebrew-versions,coeligena/homebrew-verscustomized,Ashton-W/homebrew-versions,DomT4/homebrew-versions