AppVeyor: add MSYS2 builds

Build and distcheck for MSYS2's i686 and x84_64 targets.

appveyor.bat

@@ -0,0 +1,33 @@
+rem Matrix-driven Appveyor CI script for libmypaint
+rem Currently only does MSYS2 builds.
+rem https://www.appveyor.com/docs/installed-software#mingw-msys-cygwin
+rem Needs the following vars:
+rem    MSYS2_ARCH:  x86_64 or i686
+rem    MSYSTEM:  MINGW64 or MINGW32
+
+rem Set the paths appropriately
+PATH C:\msys64\%MSYSTEM%\bin;C:\msys64\usr\bin;%PATH%
+
+rem Upgrade the MSYS2 platform
+bash -lc "pacman --noconfirm --sync --refresh --refresh pacman"
+bash -lc "pacman --noconfirm --sync --refresh --refresh --sysupgrade --sysupgrade"
+
+rem Install required tools
+bash -xlc "pacman --noconfirm -S --needed base-devel"
+
+rem Install the relevant native dependencies
+bash -xlc "pacman --noconfirm -S --needed mingw-w64-%MSYS2_ARCH%-json-c"
+bash -xlc "pacman --noconfirm -S --needed mingw-w64-%MSYS2_ARCH%-glib2"
+bash -xlc "pacman --noconfirm -S --needed mingw-w64-%MSYS2_ARCH%-gobject-introspection"
+
+rem Invoke subsequent bash in the build tree
+cd %APPVEYOR_BUILD_FOLDER%
+set CHERE_INVOKING=yes
+
+rem Build/test scripting
+bash -xlc "set pwd"
+bash -xlc "env"
+
+bash -xlc "./autogen.sh"
+bash -xlc "./configure"
+bash -xlc "make distcheck"

appveyor.yml

@@ -1,6 +1,11 @@
+environment:
+  matrix:
+    - MSYS2_ARCH: x86_64
+      MSYSTEM: MINGW64
+    - MSYS2_ARCH: i686
+      MSYSTEM: MINGW32
+
 build_script:
-    - SET PATH=%PATH%;c:\MinGW\bin
-    - gcc -o minimal examples/minimal.c -std=c99 -lm -I.
-test_script:
-    - minimal
+  - '%APPVEYOR_BUILD_FOLDER%\appveyor.bat'
+
 deploy: off