fix giflib4 detection

and a problem in giflib4 error handling

ChangeLog

@@ -5,6 +5,7 @@
 - add giflib5 support
 - allow resize >1 on one axis, <1 on the other
 - vips_resize has an optional @kernel argument
+- fix giflib4 detection [felixbuenemann]
 
 29/1/16 started 8.3
 - add vips_reduce*() ... a fast path for affine downsize

acinclude.m4

@@ -631,14 +631,14 @@ if test "$GIFLIB_LIBS" = ""; then
   INCLUDES="$GIFLIB_INCLUDES $INCLUDES"
 
   # Try the standard search path first
-  AC_TRY_LINK([#include <gif_lib.h>],[EGifSetGifVersion(0,0)], [
+  AC_TRY_LINK([#include <gif_lib.h>],[DGifSlurp(0)], [
     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(0,0)], [
+    AC_TRY_LINK([#include <gif_lib.h>],[DGifSlurp(0)], [
       GIFLIB_LIBS="-L${prefix}/lib -lgif"
     ], [
       GIFLIB_LIBS=no

libvips/foreign/gifload.c

@@ -160,7 +160,7 @@ vips_foreign_load_gif_error_vips( VipsForeignLoadGif *gif, int error )
 
 	const char *message;
 
-	if( (message = vips_foreign_load_gif_errstr( gif->file->Error )) )
+	if( (message = vips_foreign_load_gif_errstr( error )) )
 		vips_error( class->nickname, "%s", message ); 
 }