configure.ac: conditionally define HAVE_X11

configure.ac

@@ -891,6 +891,7 @@ AC_PATH_XTRA
 if test x$x_includes != xNONE; then
 	AC_DEFINE(STARPU_HAVE_X11, [1], [enable X11])
 fi
+AM_CONDITIONAL(HAVE_X11, test x$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