Include gif image decoding in images/ on linux.

tools/skimage_main.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBitmap.h"
 #include "SkCommandLineFlags.h"
 #include "SkGraphics.h"
@@ skipping 141 matching lines @@
     print_strings("Missing codec", nocodecs);
     print_strings("Failed to decode", failures);
     print_strings("Decoded", successes);
 
     return 0;
 }
 
 void forceLinking();
 
 void forceLinking() {
+    // This function leaks, but that is okay because it is not intended
+    // to be called. It is only here so that the linker will include the
+    // decoders.
     SkDEBUGCODE(SkImageDecoder *creator = ) CreateJPEGImageDecoder();
     SkASSERT(creator);
+#ifdef SK_BUILD_FOR_UNIX
+    SkDEBUGCODE(creator = ) CreateGIFImageDecoder();
+    SkASSERT(creator);
+#endif
 }
 
 #if !defined SK_BUILD_FOR_IOS
 int main(int argc, char * const argv[]) {
     return tool_main(argc, (char**) argv);
 }
 #endif