try and only look for giflib4

see https://github.com/jcupitt/libvips/issues/407

README.md

@@ -187,6 +187,8 @@ If available, libvips adds support for EXIF metadata in JPEG files.
 The standard gif loader. If this is not present, vips will try to load gifs
 via imagemagick instead.
 
+vips will only work with giflib 4. 
+
 ### librsvg
 
 The usual SVG loader. If this is not present, vips will try to load SVGs

acinclude.m4

@@ -631,14 +631,16 @@ if test "$GIFLIB_LIBS" = ""; then
   INCLUDES="$GIFLIB_INCLUDES $INCLUDES"
 
   # Try the standard search path first
-  AC_TRY_LINK([#include <gif_lib.h>],[EGifSetGifVersion("")], [
+  # look for GifLastError() since it was removed for giflib5 and we only
+  # (for now) work with giflib4
+  AC_TRY_LINK([#include <gif_lib.h>],[GifLastError()], [
     GIFLIB_LIBS="-lgif"
   ], [
     # giflib is not in the standard search path, try $prefix
 
     LIBS="-L${prefix}/lib $LIBS"
 
-    AC_TRY_LINK([#include <gif_lib.h>],[EGifSetGifVersion("")], [
+    AC_TRY_LINK([#include <gif_lib.h>],[GifLastError()], [
       GIFLIB_LIBS="-L${prefix}/lib -lgif"
     ], [
       GIFLIB_LIBS=no

configure.ac

@@ -964,6 +964,7 @@ file import/export with libpng: 	$with_png
   (requires libpng-1.2.9 or later)
 file import/export with libtiff:	$with_tiff
 file import/export with giflib:		$with_giflib
+  (requires libgif-4.x)
 file import/export with libjpeg:	$with_jpeg
 image pyramid export:			$with_gsf
   (requires libgsf-1 1.14.27 or later)