Add depexts for pkgconf for MSYS2

Cygwin uses pkgconf --personality, but MSYS2 uses native builds.

packages/conf-mingw-w64-pkgconf-i686/conf-mingw-w64-pkgconf-i686.1/opam

@@ -0,0 +1,14 @@
+opam-version: "2.0"
+synopsis: "Virtual package for pkgconf on i686 mingw-w64 (32-bit x86)"
+description: "Ensures the i686 version of pkgconf for the mingw-w64 project is available"
+maintainer: "David Allsopp <david@tarides.com>"
+authors: "Ariadne Conill et al"
+license: "ISC"
+homepage: "http://pkgconf.org"
+bug-reports: "https://github.com/ocaml/opam-repository/issues"
+flags: conf
+available: os = "win32"
+build: ["i686-w64-mingw32-pkgconf" "--version"]
+depexts: [
+  ["mingw-w64-i686-pkgconf"] {os = "win32" & os-distribution = "msys2"}
+]

packages/conf-mingw-w64-pkgconf-x86_64/conf-mingw-w64-pkgconf-x86_64.1/opam

@@ -0,0 +1,14 @@
+opam-version: "2.0"
+synopsis: "Virtual package for pkgconf on x86_64 mingw-w64 (64-bit x86_64)"
+description: "Ensures the x86_64 version of pkgconf for the mingw-w64 project is available"
+maintainer: "David Allsopp <david@tarides.com>"
+authors: "Ariadne Conill et al"
+license: "ISC"
+homepage: "http://pkgconf.org"
+bug-reports: "https://github.com/ocaml/opam-repository/issues"
+flags: conf
+available: os = "win32"
+build: ["x86_64-w64-mingw32-pkgconf" "--version"]
+depexts: [
+  ["mingw-w64-x86_64-pkgconf"] {os = "win32" & os-distribution = "msys2"}
+]