egl: Only enable GLX backend if X11 EGL platform is enabled

We don't want to compile in a bunch of X11 dependencies in libEGL if
we can't run EGL on X11.

configure.ac

@@ -1273,10 +1273,6 @@ if test "x$enable_egl" = xyes; then
 
     if test "$enable_static" != yes; then
         # build egl_glx when libGL is built
-        if test "x$enable_glx" = xyes; then
-	    HAVE_EGL_DRIVER_GLX=1
-        fi
-
         PKG_CHECK_MODULES([LIBUDEV], [libudev > 150],
                           [have_libudev=yes],[have_libudev=no])
         if test "$have_libudev" = yes; then
@@ -1545,6 +1541,10 @@ for plat in $egl_platforms; do
 
 	x11)
 		PKG_CHECK_MODULES([XCB_DRI2], [x11-xcb xcb-dri2 >= 1.8 xcb-xfixes])
+
+                if test "x$enable_glx" = xyes; then
+                        HAVE_EGL_DRIVER_GLX=1
+                fi
 		;;
 
 	drm)