Merge topic 'platform-vars'

a136b6ec98 MINGW: Define variable only when targeting Windows platforms
39c5dad0cb Ninja: Remove redundant check for GNU-like compiler on Windows
0b7ae84a96 Cygwin: Remove redundant definitions of CYGWIN and UNIX variables

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: buildbot <buildbot@kitware.com>
Merge-request: !6538