Fiy Appveyor build for MinGW

The Appveyor build failed with the following error message

> CMake Error at C:/Program Files (x86)/CMake/share/cmake-3.4/Modules/CMakeMinGWFindMake.cmake:22 (message):
>  sh.exe was found in your PATH, here:
>
>  C:/Program Files/Git/usr/bin/sh.exe
>
>  For MinGW make to work correctly sh.exe must NOT be in your path.
>
>  Run cmake from a shell that does not have sh.exe in your PATH.
>
>  If you want to use a UNIX shell, then use MSYS Makefiles.

A possible fix was found here:
http://help.appveyor.com/discussions/problems/3193-cmake-building-for-mingw-issue-with-git-shexe

The path to sh.exe has been removed from the Windows PATH variable.

appveyor.yml

@@ -106,6 +106,11 @@ install:
   
 
 before_build:
+  # Remove sh.exe from the path otherwise CMake will complain:
+  # "sh.exe was found in your PATH, here: C:/Program Files/Git/usr/bin/sh.exe"
+  # and the MinGW build will not work (the Visual Studio build does not care).
+  # See http://help.appveyor.com/discussions/problems/3193-cmake-building-for-mingw-issue-with-git-shexe
+  - set PATH=%PATH:C:\Program Files\Git\usr\bin;=%
   # Set up mingw commands
   - if "%compiler_name%"=="gcc" (set "generator=MinGW Makefiles")
   - if "%compiler_name%"=="gcc" (set "build=mingw32-make -j4")