Fix cases where GIF_LIB_VERSION is not defined

Versions up to 4.1.6 of GIFLIB defined a GIF_LIB_VERSION macro that was string-valued.
Versions after 4.1.6 define integer-valued GIFLIB_MAJOR, GIFLIB_MINOR, and GIFLIB_RELEASE macros for the three components of the version.

Check for GIF_LIB_VERSION, otherwise use the new macros.

src/init.cc

@@ -51,9 +51,16 @@ init (Handle<Object> target) {
   }
   target->Set(String::New("jpegVersion"), String::New(jpeg_version));
 #endif
+
 #ifdef HAVE_GIF
+#ifndef GIF_LIB_VERSION
+  char gif_version[10];
+  snprintf(gif_version, 10, "%d.%d.%d", GIFLIB_MAJOR, GIFLIB_MINOR, GIFLIB_RELEASE);
+  target->Set(String::New("gifVersion"), String::New(gif_version));
+#else
   target->Set(String::New("gifVersion"), String::New(GIF_LIB_VERSION));
 #endif
+#endif
 }
 
 NODE_MODULE(canvas,init);