appveyor: Install pkg-config and small improvements

Author: Mitsutoshi Aoe

appveyor.yml

@@ -2,19 +2,21 @@ platform: x64
 shallow_clone: true
 environment:
   global:
-    STACK_ROOT: "c:\\sr"
+    STACK_ROOT: c:\sr
 cache:
-- "c:\\sr -> appveyor.yml"
+- c:\sr -> appveyor.yml
+- c:\msys64\var\cache\pacman\pkg -> appveyor.yml
 install:
 - set HOME=.
-- set "PATH=C:\msys64\usr\bin;%PATH%"
+- set "PATH=c:\msys64\usr\bin;%PATH%"
 - curl -ostack.zip -LsS --insecure https://www.stackage.org/stack/windows-x86_64
 - 7z x stack.zip stack.exe
 - stack setup > nul
 - stack exec -- pacman --noconfirm --needed -Sy bash pacman pacman-mirrors msys2-runtime msys2-runtime-devel
 - stack exec -- pacman --noconfirm -Syu
 - stack exec -- pacman --noconfirm -Syuu
-- stack exec -- pacman --noconfirm -S base-devel mingw-w64-x86_64-toolchain mingw-w64-x86_64-gtk2
+- stack exec -- pacman --noconfirm -S base-devel mingw-w64-x86_64-pkg-config mingw-w64-x86_64-toolchain mingw-w64-x86_64-gtk2
+- stack exec -- pacman --noconfirm -Sc
 build_script:
 - stack --no-terminal build --only-dependencies
 test_script: