CMake: Support external callback in configuration

An external CMake script can be used to debug CMake code, modify/read
target properties, inter-target dependencies, non-cache variables etc.

Reviewed by: LazyDodo, brecht
Differential Revision: https://developer.blender.org/D13830

CMakeLists.txt

@@ -711,6 +711,9 @@ endif()
 set(POSTINSTALL_SCRIPT "" CACHE FILEPATH "Run given CMake script after installation process")
 mark_as_advanced(POSTINSTALL_SCRIPT)
 
+set(POSTCONFIGURE_SCRIPT "" CACHE FILEPATH "Run given CMake script as the last step of CMake configuration")
+mark_as_advanced(POSTCONFIGURE_SCRIPT)
+
 # end option(...)
 
 
@@ -2074,3 +2077,8 @@ endif()
 if(0)
   print_all_vars()
 endif()
+
+# Should be the last step of configuration.
+if(POSTCONFIGURE_SCRIPT)
+  include(${POSTCONFIGURE_SCRIPT})
+endif()