Fix FONTFORGE_ARG_WITH_CAIRO() to hard-stop if "--with" but no cairo

Some operating systems have a weaker pkg-config, and won't find CAIRO if
looking only for a package. Therefore, try look for the package first,
and then if failed, try looking for library and header files.
Check for silly options like trying to specify with Cairo, but no png.
minor alphabetization of options. Remove some dead code for *.pc
Merge pull request #1509 from JoesCat/cairo_or_halt

Cairo or halt