Add basic version check for giflib

This is used e.g. in KDE, there on Solaris an old version of giflib
was found (version 3), which could be detected by checking for the
UserData member variable:
http://websvn.kde.org/?view=revision&revision=545738

Alex

Modules/FindGIF.cmake

@@ -1,7 +1,11 @@
-# This module defines
+# This module searches giflib and defines
 # GIF_LIBRARIES - libraries to link to in order to use GIF
 # GIF_FOUND, if false, do not try to link
 # GIF_INCLUDE_DIR, where to find the headers
+# GIF_VERSION, reports either version 4 or 3 (for everything before version 4)
+#
+# The minimum required version of giflib can be specified using the
+# standard syntax, e.g. FIND_PACKAGE(GIF 4)
 #
 # $GIF_DIR is an environment variable that would
 # correspond to the ./configure --prefix=$GIF_DIR
@@ -47,9 +51,30 @@ FIND_LIBRARY(GIF_LIBRARY
 # see readme.txt
 SET(GIF_LIBRARIES ${GIF_LIBRARY})
 
+# Very basic version detection.
+# The GIF_LIB_VERSION string in gif_lib.h seems to be unreliable, since it seems
+# to be always " Version 2.0, " in versions 3.x of giflib.
+# In version 4 the member UserData was added to GifFileType, so we check for this
+# one.
+# http://giflib.sourcearchive.com/documentation/4.1.4/files.html
+IF(GIF_INCLUDE_DIR)
+  INCLUDE(CMakePushCheckState)
+  INCLUDE(CheckStructHasMember)
+  CMAKE_PUSH_CHECK_STATE()
+  SET(GIF_VERSION 3)
+  SET(CMAKE_REQUIRED_INCLUDES "${GIF_INCLUDE_DIR}")
+  CHECK_STRUCT_HAS_MEMBER(GifFileType UserData gif_lib.h GIF_GifFileType_UserData )
+  IF(GIF_GifFileType_UserData)
+    SET(GIF_VERSION 4)
+  ENDIF()
+  CMAKE_POP_CHECK_STATE()
+ENDIF()
+
+
 # handle the QUIETLY and REQUIRED arguments and set GIF_FOUND to TRUE if
 # all listed variables are TRUE
 INCLUDE(${CMAKE_CURRENT_LIST_DIR}/FindPackageHandleStandardArgs.cmake)
-FIND_PACKAGE_HANDLE_STANDARD_ARGS(GIF  DEFAULT_MSG  GIF_LIBRARY  GIF_INCLUDE_DIR)
+FIND_PACKAGE_HANDLE_STANDARD_ARGS(GIF  REQUIRED_VARS  GIF_LIBRARY  GIF_INCLUDE_DIR
+                                       VERSION_VAR GIF_VERSION )
 
 MARK_AS_ADVANCED(GIF_INCLUDE_DIR GIF_LIBRARY)