RFE: Report changes to cached values

This issue was created automatically from an original Mantis Issue. Further discussion may take place here.

Original description:

Add a variable to CMake such that it will report when a cached variable's value is different from the default being set. For example:

set(var dflt CACHE STRING "description")

could output:

path/to/CMakeLists.txt:42: var 'dflt' -> 'cachedvalue'

and could also, at the end of configure, print out a command line (or initial cache file) to be used to duplicate the build on another machine. I was thinking of CMAKE_REPORT_DELTA as the control variable.

Came up from a gn thread.

Comment from @brad.king:

Note that in Modules/CMakeGenericSystem.cmake we do:

if(NOT DEFINED CMAKE_INSTALL_PREFIX)
  set(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT 1)
endif()
# ...
  set(CMAKE_INSTALL_PREFIX "/usr/local" CACHE PATH "...")
# ...

so that applications can know when they can force a different default for CMAKE_INSTALL_PREFIX without overriding a user setting.

This could be generalized for use with all cache entries. It would be helpful for projects that want to override the default CMAKE_C_FLAGS_DEBUG, for example, without needing to use CMAKE_USER_MAKE_RULES_OVERRIDE and a separate file.