ci: attempt to setup msys2 build

Likely won't work, but it's a start.

.github/workflows/build-and-test.yml

@@ -14,6 +14,7 @@ jobs:
         os:
           - ubuntu-latest
           - macos-latest
+          - windows-latest
 
     steps:
       - uses: actions/checkout@v2
@@ -29,6 +30,15 @@ jobs:
         run: |
           brew install meson ninja libgpg-error libtool pkg-config glib gmime xapian
 
+      - if: contains(matrix.os, 'windows')
+        uses: msys2/setup-msys2@v2
+        with:
+          msystem: MINGW64
+          update: true
+        name: windows-deps
+        run: |
+          pacman -S base-devel msys-devel gcc git meson mingw-w64-gmime mingw-w64-xapian-core
+
       - name: configure
         run: ./autogen.sh -Dguile=disabled