Adds --disable-npapi as an additional build option. The default is still to build in NPAPI compatibility, but with this flag, it's finally possible to build without pulling in GTK2 on systems that don't have the package. This essentially ifdefs everything associated with NPAPI, following various Mozilla removal bugs. Ref: BZ 1682030, BZ 1251202, BZ 1217665, BZ 1687239