#854 npambulant cannot be build for gtk3

[Kees Blom]

Although npambulant has been adapted to support gtk3, it cannot be properly tested
since Firefox handles only gtk2 compatible npapi plugins. The firefox port to gt3
is still open, and the current mercurial build crashes when opening a gtk3 compatible npapi plugin. (see: https://bugzilla.mozilla.org/show_bug.cgi?id=627699)