configure.ac: loosen --enable-glvnd check to honour egl

Currently we error out when building GLVND w/o GLX.

That was the original premice before we had EGL. As the commit says,
that error should be reworked to honour both - do so.

v2: Drop noop *);; (Eric)

Reported-by: Lukas Rusak <lorusak@gmail.com>
Fixes: ce562f9e3f ("EGL: Implement the libglvnd interface for EGL (v3)")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Tested-by: Lukas Rusak <lorusak@gmail.com> (v1)
(cherry picked from commit b4967561c0)

configure.ac

@@ -1416,18 +1416,10 @@ AC_ARG_ENABLE([libglvnd],
 AM_CONDITIONAL(USE_LIBGLVND, test "x$enable_libglvnd" = xyes)
 
 if test "x$enable_libglvnd" = xyes ; then
-    dnl XXX: update once we can handle more than libGL/glx.
-    dnl Namely: we should error out if neither of the glvnd enabled libraries
-    dnl are built
     case "x$enable_glx" in
-    xno)
-        AC_MSG_ERROR([cannot build libglvnd without GLX])
-        ;;
     xxlib | xgallium-xlib )
         AC_MSG_ERROR([cannot build libgvnd when Xlib-GLX or Gallium-Xlib-GLX is enabled])
         ;;
-    xdri)
-        ;;
     esac
 
     PKG_CHECK_MODULES([GLVND], libglvnd >= 0.2.0)
@@ -1436,6 +1428,10 @@ if test "x$enable_libglvnd" = xyes ; then
 
     DEFINES="${DEFINES} -DUSE_LIBGLVND=1"
     DEFAULT_GL_LIB_NAME=GLX_mesa
+
+    if test "x$enable_glx" = xno -a "x$enable_egl" = xno; then
+        AC_MSG_ERROR([cannot build libglvnd without GLX or EGL])
+    fi
 fi
 
 AC_ARG_WITH([gl-lib-name],