Attempt to fix GCC for appveyor

appveyor.yml

@@ -23,6 +23,17 @@ install:
   - appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
   - rustup-init -yv --default-toolchain %channel% --default-host %target%
   - set PATH=%PATH%;%USERPROFILE%\.cargo\bin
+  - ps: |
+      if ($env:target -contains "gnu" -And $env:PLATFORM -eq "x64") {
+          Start-FileDownload "http://sourceforge.net/projects/mingw-w64/files/Toolchains%20targetting%20Win64/Personal%20Builds/mingw-builds/5.1.0/threads-win32/seh/x86_64-5.1.0-release-win32-seh-rt_v4-rev0.7z/download" -FileName mingw64.7z
+          7z x -oC:\ mingw64.7z > $null
+          $env:path = "C:\mingw64\bin;" + $env:path
+          gcc --version
+      }
+      elseif ($env:target -contains "gnu") {
+          $env:path = "C:\mingw\bin;" + $env:path
+          gcc --version
+      }
   - rustc -vV
   - cargo -vV