meson: Basic cygwin support
There likely are further issues, but as evidenced by the CI task proposed by
Justin in the referenced thread, this suffices to build and run basic tests in
cygwin (some fixes for the test infrastructure are needed, but that's
independent of the meson aspect).
Author: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/
20221021034040.GT16921@telsasoft.com