mesa: don't install GLX files if GLX is not built

Cc: "11.2 12.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Akihiko Odaki <akihiko.odaki.4i@stu.hosei.ac.jp>
[Emil Velikov: Drop guards around dri_interface.h, add stable tag]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
diff --git a/configure.ac b/configure.ac
index c2525d3..367e9b5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1060,6 +1060,7 @@
     ;;
 esac
 
+AM_CONDITIONAL(HAVE_GLX, test "x$enable_glx" != xno)
 AM_CONDITIONAL(HAVE_DRI_GLX, test "x$enable_glx" = xdri)
 AM_CONDITIONAL(HAVE_XLIB_GLX, test "x$enable_glx" = xxlib)
 AM_CONDITIONAL(HAVE_GALLIUM_XLIB_GLX, test "x$enable_glx" = xgallium-xlib)