FindCygwin: Fix regression when CYGWIN_INSTALL_PATH is already set

Tibor Szaborequested to merge
szabootibor/cmake:release into master

The change in !883 (merged) 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.

Topic-rename: FindCygwin-fix-regression

Edited by Brad King

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