mesa: Remove pointless USE_EXTERNAL_DXTN_LIB macro.

I'm not sure what was the original intention, but currently
USE_EXTERNAL_DXTN_LIB always ends up defined, one way or another.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>

configure.ac

@@ -230,7 +230,7 @@ _SAVE_LDFLAGS="$LDFLAGS"
 _SAVE_CPPFLAGS="$CPPFLAGS"
 
 dnl Compiler macros
-DEFINES="-DUSE_EXTERNAL_DXTN_LIB=1"
+DEFINES=""
 AC_SUBST([DEFINES])
 case "$host_os" in
 linux*|*-gnu*|gnu*)

src/mesa/main/texcompress_s3tc.c

@@ -29,10 +29,6 @@
  * GL_EXT_texture_compression_s3tc support.
  */
 
-#ifndef USE_EXTERNAL_DXTN_LIB
-#define USE_EXTERNAL_DXTN_LIB 1
-#endif
-
 #include "glheader.h"
 #include "imports.h"
 #include "dlopen.h"
@@ -76,7 +72,6 @@ _mesa_init_texture_s3tc( struct gl_context *ctx )
 {
    /* called during context initialization */
    ctx->Mesa_DXTn = GL_FALSE;
-#if USE_EXTERNAL_DXTN_LIB
    if (!dxtlibhandle) {
       dxtlibhandle = _mesa_dlopen(DXTN_LIBNAME, 0);
       if (!dxtlibhandle) {
@@ -117,9 +112,6 @@ _mesa_init_texture_s3tc( struct gl_context *ctx )
    if (dxtlibhandle) {
       ctx->Mesa_DXTn = GL_TRUE;
    }
-#else
-   (void) ctx;
-#endif
 }
 
 /**