Autodetect libgif so extension and try to load it.

diff --git a/configure.ac b/configure.ac
index 62c3c00..fdfc6d5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1028,6 +1028,7 @@
   WINE_GET_SONAME(curses,waddch)
   WINE_GET_SONAME(jpeg,jpeg_start_decompress)
   WINE_GET_SONAME(ungif,DGifOpen)
+  WINE_GET_SONAME(gif,DGifOpen)
 fi