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.