FindGIF: Modernize

This brings the module up to the current find module standards by adding
the GIF_INCLUDE_DIRS variable and the target GIF::GIF. It also updates
the documentation style to that of similar modules.

Modules/FindGIF.cmake

@@ -7,22 +7,43 @@ FindGIF
 
 This finds the GIF library (giflib)
 
-The module defines the following variables:
+Imported targets
+^^^^^^^^^^^^^^^^
+
+This module defines the following :prop_tgt:`IMPORTED` target:
+
+``GIF::GIF``
+  The giflib library, if found.
+
+Result variables
+^^^^^^^^^^^^^^^^
+
+This module will set the following variables in your project:
 
 ``GIF_FOUND``
-  True if giflib was found
+  If false, do not try to use GIF.
+``GIF_INCLUDE_DIRS``
+  where to find gif_lib.h, etc.
 ``GIF_LIBRARIES``
-  Libraries to link to in order to use giflib
-``GIF_INCLUDE_DIR``
-  where to find the headers
+  the libraries needed to use GIF.
 ``GIF_VERSION``
-  3, 4 or a full version string (eg 5.1.4) for versions >= 4.1.6
+  3, 4 or a full version string (eg 5.1.4) for versions >= 4.1.6.
+
+Cache variables
+^^^^^^^^^^^^^^^
+
+The following cache variables may also be set:
 
-The minimum required version of giflib can be specified using the
-standard syntax, e.g.  find_package(GIF 4)
+``GIF_INCLUDE_DIR``
+  where to find the GIF headers.
+``GIF_LIBRARY``
+  where to find the GIF library.
+
+Hints
+^^^^^
 
-$GIF_DIR is an environment variable that would correspond to the
-./configure --prefix=$GIF_DIR
+``GIF_DIR`` is an environment variable that would correspond to the
+``./configure --prefix=$GIF_DIR``.
 #]=======================================================================]
 
 # Created by Eric Wing.
@@ -44,9 +65,6 @@ find_library(GIF_LIBRARY
   PATH_SUFFIXES lib
 )
 
-# see readme.txt
-set(GIF_LIBRARIES ${GIF_LIBRARY})
-
 # Very basic version detection.
 # The GIF_LIB_VERSION string in gif_lib.h seems to be unreliable, since it seems
 # to be always " Version 2.0, " in versions 3.x of giflib.
@@ -90,4 +108,20 @@ include(${CMAKE_CURRENT_LIST_DIR}/FindPackageHandleStandardArgs.cmake)
 FIND_PACKAGE_HANDLE_STANDARD_ARGS(GIF  REQUIRED_VARS  GIF_LIBRARY  GIF_INCLUDE_DIR
                                        VERSION_VAR GIF_VERSION )
 
+if(GIF_FOUND)
+  set(GIF_INCLUDE_DIRS "${GIF_INCLUDE_DIR}")
+  set(GIF_LIBRARIES ${GIF_LIBRARY})
+
+  if(NOT TARGET GIF::GIF)
+    add_library(GIF::GIF UNKNOWN IMPORTED)
+    set_target_properties(GIF::GIF PROPERTIES
+      INTERFACE_INCLUDE_DIRECTORIES "${GIF_INCLUDE_DIRS}")
+    if(EXISTS "${GIF_LIBRARY}")
+      set_target_properties(GIF::GIF PROPERTIES
+        IMPORTED_LINK_INTERFACE_LANGUAGES "C"
+        IMPORTED_LOCATION "${GIF_LIBRARY}")
+    endif()
+  endif()
+endif()
+
 mark_as_advanced(GIF_INCLUDE_DIR GIF_LIBRARY)