Add depexts for conf-g++ for mingw-w64

packages/conf-mingw-w64-g++-i686/conf-mingw-w64-g++-i686.1/opam

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+opam-version: "2.0"
+synopsis: "Virtual package for g++ on i686 mingw-w64 (32-bit x86)"
+description: "Ensures the i686 version of g++ from the mingw-w64 project is available"
+maintainer: "David Allsopp <david@tarides.com>"
+authors: "David Allsopp"
+license: "GPL-3.0-or-later"
+homepage: "https://www.mingw-w64.org"
+bug-reports: "https://github.com/ocaml/opam-repository/issues"
+flags: conf
+available: os = "win32"
+build: ["i686-w64-mingw32-g++" "--version"]
+depends: ["conf-mingw-w64-gcc-i686" {build}]
+depexts: [
+  ["mingw64-i686-gcc-g++"] {os = "win32" & os-distribution = "cygwin"}
+]

packages/conf-mingw-w64-g++-x86_64/conf-mingw-w64-g++-x86_64.1/opam

@@ -0,0 +1,15 @@
+opam-version: "2.0"
+synopsis: "Virtual package for g++ on x86_64 mingw-w64 (64-bit x86_64)"
+description: "Ensures the x86_64 version of g++ from the mingw-w64 project is available"
+maintainer: "David Allsopp <david@tarides.com>"
+authors: "David Allsopp"
+license: "GPL-3.0-or-later"
+homepage: "https://www.mingw-w64.org"
+bug-reports: "https://github.com/ocaml/opam-repository/issues"
+flags: conf
+available: os = "win32"
+build: ["x86_64-w64-mingw32-g++" "--version"]
+depends: ["conf-mingw-w64-gcc-x86_64" {build}]
+depexts: [
+  ["mingw64-x86_64-gcc-g++"] {os = "win32" & os-distribution = "cygwin"}
+]