cmake_policy: Add undocumented GET_WARNING command

This command is intended for modules that issue policy warnings so
they can get the warning string from CMake in a uniform manner,
rather than duplicating the string. Several modules been updated
to include an example of the usage of this new command.
Status Job ID Name Coverage
  External
passed cdash

failed cmake-dash3win7-windows-release-ninja

passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-hythloth-linux-release-lint-iwyu-tidy

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-release-makefiles

passed cmake-terfin-windows-release-vs2017

passed ghostflow-branch-check-master

passed ghostflow-commit-check

passed ghostflow-stager

passed cdash

failed cmake-dash3win7-windows-release-ninja

01:12:17

passed cmake-hythloth-linux-debug-makefiles+sphinx

00:56:51

passed cmake-hythloth-linux-release-lint-iwyu-tidy

00:17:56

passed cmake-moonshade-osx-debug-ninja

00:10:58

passed cmake-moonshade-osx-release-makefiles

00:12:54

passed cmake-terfin-windows-release-vs2017

00:34:56

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager