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
This should ideally not be needed on Mac but compilation fails due to
sizeof `DxgiAdapterDesc` not known.
Didn't face this issue on Linux though, probably due to a different include
chain which already has this file included somewhere along the way.
This creates a number of stubs and leaves some surrounding code that may be irrelevant (eg. recorded time stamps, status variables).
Stub resolution/removal should be a follow-up to this.