Log Message:
xulrunner192: fix build on netbsd-current

This package uses a cdefs.h that originated out of BSD so redefines a lot
of macros normally in BSD's cdefs.h. avoid redefining if those macros
already exist.

Adjust GCC version check to match intention, it was GCC < 2.5 without
__attribute__, not every GCC with a minor below 5!

Patch from Chavdar Ivanov on pkgsrc-users, adjusted to be more conservative
by myself (it's likely some operating systems do need __BEGIN_DECLS!)

PKGREVISION->38 out of paranoia