PR #3133 caused build breakage on non-gtk platforms by unconditionally
including the header (because the def defaults to true in configure)
This also reorganizes the configure script to add a check for
MOZ_WIDGET_GTK (hoisted up a little) so the new configure entry is
simply ignored on non-GTK targets.
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