Add configuration for autoconf under cygwin

Author: Basile Desloges

packages/conf-autoconf/conf-autoconf.0.2/opam

@@ -5,7 +5,8 @@ authors: "https://www.gnu.org/software/autoconf/autoconf.html#maintainer"
 bug-reports: "https://github.com/ocaml/opam-repository/issues"
 license: "GPL-3.0-only"
 build: [
-  ["autoconf" "-V"]
+  ["sh" "-exc" "autoconf -V"] {os = "win32" & os-distribution != "cygwinports"}
+  ["autoconf" "-V"] {os != "win32" | os-distribution = "cygwinports"}
 ]
 depexts: [
   ["autoconf"] {os-family = "debian" | os-family = "ubuntu"}