config: use xproto pkgconfig includex11dir variable

Rather than appending X11 to the include dir.
It should be safe to use as it has been added in 2005.

Use a local variable name matching the pkgconfig name.

Reviewed-by: Jeremy Huddleston <jeremyhu@apple.com>
Signed-off-by: Gaetan Nadon <memsize@videotron.ca>
1 file changed