revert part of r338: X_LIBS could be empty, while x_includes is documented to be non-empty.

configure.ac

@@ -888,10 +888,10 @@ AC_SUBST(STARPU_OPENGL_RENDER, $enable_opengl_render)
 AC_MSG_RESULT($enable_opengl_render)
 
 AC_PATH_XTRA
-if test "x$X_LIBS" != "x"; then
+if test "x$x_includes" != "xNONE"; then
 	AC_DEFINE(STARPU_HAVE_X11, [1], [enable X11])
 fi
-AM_CONDITIONAL([HAVE_X11], [test "x$X_LIBS" != "x"])
+AM_CONDITIONAL([HAVE_X11], [test "x$x_includes" != "xNONE"])
 
 # In case there are BLAS kernels that are used by the example applications
 # we may specify which library to use. Note that this is not used for StarPU