Makefile: Only check for pkg-config when on a system that uses it
Makefile: Only check for pkg-config when on a system that uses it

This fixes compilation on OSX where pkg-config is not installed
by default.

jjthrash/kakoune,jjthrash/kakoune,Somasis/kakoune,danr/kakoune,jkonecny12/kakoune,occivink/kakoune,jkonecny12/kakoune,casimir/kakoune,alexherbo2/kakoune,mawww/kakoune,danr/kakoune,alexherbo2/kakoune,Somasis/kakoune,jjthrash/kakoune,lenormf/kakoune,occivink/kakoune,alexherbo2/kakoune,occivink/kakoune,mawww/kakoune,danr/kakoune,casimir/kakoune,casimir/kakoune,alexherbo2/kakoune,Somasis/kakoune,occivink/kakoune,danr/kakoune,jjthrash/kakoune,mawww/kakoune,mawww/kakoune,casimir/kakoune,jkonecny12/kakoune,lenormf/kakoune,Somasis/kakoune,jkonecny12/kakoune,lenormf/kakoune,lenormf/kakoune