Historically these were both added for the Makefile and Visual Studio
generators, respectively. Later the VS generators started using the
CMAKE_MAKE_PROGRAM cache entry to find the IDE build tool, and the
CMAKE_BUILD_TOOL was simply set as an alias.
Fix the documentation to explain that CMAKE_MAKE_PROGRAM is the modern
variable and that CMAKE_BUILD_TOOL is the compatibility alias, not the
other way around. Replace uses of CMAKE_BUILD_TOOL with
CMAKE_MAKE_PROGRAM in CMake-provided modules. Nothing needs to lookup
CMAKE_BUILD_TOOL in the cache, so simply set it as a normal variable.
Move the ErrorsOFF/ON common logic from CMakeLists.txt into an
ErrorsCommon file to allow other test cases to be added that do not use
the Errors test logic.
Brad King