Use pre-installed MinGW on Appveyor

Currently out Appveyor CI downloads and stashes a custom MinGW installation.
However the builder already provides both 64 and 32 bit installations of MinGW.
This patch changes our CI to use those instead.

I'm hoping this will fix issues where the g++ is broken due to the Appveyor
package caching semantics.

appveyor.yml

@@ -20,20 +20,19 @@ environment:
 
     - compiler: gcc-4.9.2-posix
       generator: "MinGW Makefiles"
-      arch: i686
+      cxx_path: 'C:\MinGW\bin'
 
     - compiler: gcc-4.9.2-posix
       generator: "MinGW Makefiles"
-      arch: x86_64
+      cxx_path: 'C:\mingw-w64\i686-5.3.0-posix-dwarf-rt_v4-rev0\mingw32\bin'
 
 matrix:
   fast_finish: true
 
 install:
-  - if "%compiler%"=="gcc-4.9.2-posix" (for /f %%a in ('python mingw.py --quiet --version "4.9.2" --arch "%arch%" --threading "posix" --location "C:\mingw-builds"') do @set "compiler_path=%%a")
-  - if not "%compiler_path%"=="" (set "PATH=%PATH%;%compiler_path%")
   # git bash conflicts with MinGW makefiles
   - if "%generator%"=="MinGW Makefiles" (set "PATH=%PATH:C:\Program Files\Git\usr\bin;=%")
+  - if not "%cxx_path%"=="" (set "PATH=%PATH%;%cxx_path%")
 
 build_script:
   - md _build -Force
@@ -51,5 +50,3 @@ artifacts:
   - path: '_build/Testing/**/*.xml'
     name: test_results
 
-cache:
-  - C:\mingw-builds