MSYS variable meaning is overloaded
The MSYS
variable has long been documented to mean that one is using the MSYS Makefiles
generator.
Since !5969 (merged), the MSYS
variable is now also set when targeting the MSYS runtime environment packaged by msys2. This is equivalent to the CYGWIN
variable set when targeting the Cygwin runtime environment.
The latter meaning is more useful, so the former meaning should be removed by a policy. It is easy to test the CMAKE_GENERATOR
variable to detect the MSYS Makefiles
generator.