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>
(cherry picked from commit 49a04acca3d4b8a570b7c1468d236e65111efb69)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/links/links.mk

@@ -15,7 +15,10 @@ ifeq ($(BR2_PACKAGE_LINKS_GRAPHICS),y)
 LINKS_CONF_OPTS += --enable-graphics
 LINKS_DEPENDENCIES += libpng
 ifeq ($(BR2_PACKAGE_XLIB_LIBXT),y)
-LINKS_CONF_OPTS += --with-x
+LINKS_CONF_OPTS += \
+	--with-x \
+	--x-includes=$(STAGING_DIR)/usr/include \
+	--x-libraries=$(STAGING_DIR)/usr/lib
 LINKS_DEPENDENCIES += xlib_libXt
 else
 LINKS_CONF_OPTS += --without-x