appveyor: don't rewrite system mingw

Download mingw-w64 into our build directory and execute it there, don't
try to overwrite the system's mingw.

script/appveyor-mingw.sh

@@ -7,15 +7,17 @@ if [ "$ARCH" = "i686" ]; then
     curl -LsSO http://sourceforge.net/projects/mingw-w64/files/Toolchains%20targetting%20Win32/Personal%20Builds/mingw-builds/4.9.2/threads-win32/sjlj/$f
   fi
   7z x $f > /dev/null
-  mv mingw32 /MinGW
+  export PATH=`pwd`/mingw32/bin:$PATH
 else
   f=x86_64-4.9.2-release-win32-seh-rt_v3-rev1.7z
   if ! [ -e $f ]; then
     curl -LsSO http://sourceforge.net/projects/mingw-w64/files/Toolchains%20targetting%20Win64/Personal%20Builds/mingw-builds/4.9.2/threads-win32/seh/$f
   fi
   7z x $f > /dev/null
-  mv mingw64 /MinGW
+  export PATH=`pwd`/mingw64/bin:$PATH
 fi
 cd build
+gcc --version
+cmake --version
 cmake -D ENABLE_TRACE=ON -D BUILD_CLAR=ON .. -G"$GENERATOR"
 cmake --build . --config RelWithDebInfo