Tibor Szabo authored Aug 11, 2017 and Brad King committed Aug 15, 2017

The change in commit v3.9.0-rc1~54^2 (FindCygwin: Use find_program
instead of find_path, 2017-05-18) broke cases when `CYGWIN_INSTALL_PATH`
is already set, e.g. on the command-line or by an earlier call to
`find_package(Cygwin)`.  Since `find_program` now finds the actual
`cygwin.bat` file, use a separate cache entry to save the location
and then compute `CYGWIN_INSTALL_PATH`.  If `CYGWIN_INSTALL_PATH`
is already set, use that to avoid `find_program` having to search.