Change libX11 variables to not conflict with AC_PATH_XTRA

Change libX11 variables to not conflict with AC_PATH_XTRA

The variable X_LIBS from AC_PATH_XTRA contains only the -L searchdir
parameter and not the -lX11 to link to Xlib. Use X11 prefixed build vars
for linking with Xlib to avoid the conflict.

Signed-off-by: Dan Nicholson <dbn.lists@gmail.com>