gtk+: relax X11 dependency version

2.6.3 was chosen because Leopard's system X11 (2.1.6) is too old to
build gtk+, and it was the most recent version that is compatible with
Leopard. There are now reports that Snow Leopard's system X11 (2.3.6)
can build this, so relax the requirement.

c.f. #15928.