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
Since these are just interpreted comments, there's 0 impact on actual code.
This removes all lines that match /* vim: set(.*)tw=80: */ with S&R -- there are
a few others scattered around which will be removed manually in a second part.