Include gif image decoding in images/ on linux.

Build SkImageDecoder_libgif in images project and link against
libgif on linux.

Ensure that the GIF decoder is used in skimage.

Requires a new dependency on libgif-dev when building on linux.

Review URL: https://codereview.chromium.org/14029011

git-svn-id: http://skia.googlecode.com/svn/trunk@8627 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tools/skimage_main.cpp b/tools/skimage_main.cpp
index dc5bdd4..b486d5d 100644
--- a/tools/skimage_main.cpp
+++ b/tools/skimage_main.cpp
@@ -159,8 +159,15 @@
 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