Fix windows (cygwin) build recipe for conf-gtk3

Author: punchagan

packages/conf-gtk3/conf-gtk3.18/opam

@@ -3,7 +3,10 @@ maintainer: "[email protected]"
 authors: "The GTK Toolkit"
 homepage: "https://developer.gnome.org/"
 bug-reports: "https://github.com/garrigue/lablgtk/issues"
-build: [["pkg-config" "--short-errors" "--print-errors" "--atleast-version" "3.18" "gtk+-3.0"]]
+build: [
+  ["pkgconf" "gtk+-3.0"] {os = "win32" & os-distribution != "cygwinports"}
+  ["pkg-config" "--short-errors" "--print-errors" "--atleast-version" "3.18" "gtk+-3.0"] {os != "win32" | os-distribution != "cygwin"}
+]
 depends: ["conf-pkg-config" {build}]
 depexts: [
   ["libgtk-3-dev" "libexpat1-dev"] {os-family = "debian" | os-family = "ubuntu"}