build: clean up libpng check, and don't require 1.2
build: clean up libpng check, and don't require 1.2

jku/matchbox-keyboard,openivimobility/matchbox-keyboard,jku/matchbox-keyboard,jku/matchbox-keyboard,mwilliams03/matchbox-keyboard,mwilliams03/matchbox-keyboard,mwilliams03/matchbox-keyboard,openivimobility/matchbox-keyboard,openivimobility/matchbox-keyboard