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 is mostly ifdefs, but as you can see, Solaris is actually a lot like Linux. They're both more SysV than BSD at core, and most of the differences have more to do with Solaris not using glibc than anything else.
I still need to audit a lot of these changes and understand why they're needed and what the alternative approaches are. After this patch, most of the core functionality needed to build Solaris is here.