configure.ac: Test glide even when the user specify it

configure.ac

@@ -3851,16 +3851,24 @@ AC_ARG_ENABLE(glide,
   [  --enable-glide          Glide (3dfx) support (default disabled)])
 if test "${enable_glide}" = "yes"
 then
-  VLC_ADD_PLUGINS([glide])
-  VLC_ADD_LDFLAGS([glide],[-lglide2x -lm])
-  VLC_ADD_CPPFLAGS([glide],[-I/usr/include/glide])
+  CFLAGS_save="${CFLAGS}"
   AC_ARG_WITH(glide,
     [    --with-glide=PATH     path to libglide],
     [ if test "${with_glide}" != "no" -a -n "${with_glide}"
       then
         VLC_ADD_CPPFLAGS([glide],[-I${with_glide}/include])
         VLC_ADD_LDFLAGS([glide],[-L${with_glide}/lib])
+        CFLAGS="$CFLAGS -I${with_glide}/include"
     fi ])
+   CFLAGS="$CFLAGS -I/usr/include/glide"
+   AC_CHECK_HEADER(glide.h,[   
+      VLC_ADD_PLUGINS([glide])
+      VLC_ADD_LDFLAGS([glide],[-lglide2x -lm])
+      VLC_ADD_CPPFLAGS([glide],[-I/usr/include/glide])
+    ],[
+      AC_MSG_ERROR([You don't have libglide. Install it or do not use --enable-glide])
+     ])
+   CFAGS="${CFLAGS_save}"
 fi
 
 dnl