links: fix X11 library/includes detection

links: fix X11 library/includes detection

Fixes:
http://autobuild.buildroot.net/results/4b0/4b02713ed5c10b58f390f96ea25f2a45c9a47af4/
http://autobuild.buildroot.net/results/ac6/ac668f867b9de7a2c61d263a894480afc181dd9b/
http://autobuild.buildroot.net/results/f02/f0248b3bf4a73210f210f422c8df067a6ea28aa1/

The configure script uses AC_PATH_X, which searches the host for the X11
libraries/headers, causing build failures.

https://www.gnu.org/software/autoconf/manual/autoconf-2.65/html_node/System-Services.html

Fix it by using --x-includes / --x-libraries like we do elsewhere.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>