config: remove PKG_PROG_PKG_CONFIG test

config: remove PKG_PROG_PKG_CONFIG test

If pkg-config is missing, the configuration fails while initializing
XORG_* macros:

warning: PKG_PROG_PKG_CONFIG is m4_require'd but not m4_defun'd
line 11622: PKG_PROG_PKG_CONFIG: command not found

The output is sufficient to locate the problem.
It is not possible, or very difficult, to not have pkg-config
installed as the whole desktop and countless other software
depends on it.

Acked-by: Daniel Stone <daniel@fooishbar.org>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Gaetan Nadon <memsize@videotron.ca>