FindGLUT: Provide legacy GLUT_INCLUDE_DIR result in pkg-config code path

Since commit f90d15458a (FindGLUT: Use pkg-config to find flags if
available, 2021-06-11, v3.22.0-rc1~469^2) we return early if pkg-config
provides the information.  During review of that commit, code to
populate the legacy `GLUT_INCLUDE_DIR` result variable was removed from
that code path.  Add code to provide it.

Also fix the test case to use `GLUT_INCLUDE_DIR`, the result variable
documented officially by the module.

Fixes: #23018

Modules/FindGLUT.cmake

@@ -71,6 +71,8 @@ find_package(PkgConfig)
 if(PKG_CONFIG_FOUND)
   pkg_check_modules(GLUT glut)
   if(GLUT_FOUND)
+    # In the non-pkg-config code path we only provide GLUT_INCLUDE_DIR.
+    set(GLUT_INCLUDE_DIR "${GLUT_INCLUDE_DIRS}")
     _add_glut_target_simple()
     FIND_PACKAGE_HANDLE_STANDARD_ARGS(GLUT REQUIRED_VARS GLUT_FOUND)
     return()

Tests/FindGLUT/Test/CMakeLists.txt

@@ -9,7 +9,7 @@ target_link_libraries(testglut_tgt GLUT::GLUT)
 add_test(NAME testglut_tgt COMMAND testglut_tgt)
 
 add_executable(testglut_var main.c)
-target_include_directories(testglut_var PRIVATE ${GLUT_INCLUDE_DIRS})
+target_include_directories(testglut_var PRIVATE ${GLUT_INCLUDE_DIR})
 target_link_libraries(testglut_var PRIVATE ${GLUT_LIBRARIES})
 add_test(NAME testglut_var COMMAND testglut_var)