Be more paranoid about checking for GTK+

Be more paranoid about checking for GTK+

From time to time people run into issues where the configure script
detects GTK+ when it is either not installed, or not functional due to
a missing pixman. Most recently:

  https://bugs.freedesktop.org/show_bug.cgi?id=29736

This patch makes the configure script more paranoid by

- always using PKG_CHECK_MODULES and not PKG_CHECK_EXISTS, since it
seems PKG_CHECK_EXISTS will sometimes return true even if a dependency
of GTK+, such as pixman-1, is missing.

- explicitly checking that pixman-1 is installed before enabling GTK+.

Cc: my.somewhat.lengthy.loginname@gmail.com