fix appveyor config for gnu target

.appveyor.yml

@@ -33,6 +33,12 @@ environment:
 # Install Rust and Cargo
 # (Based on from https://github.com/rust-lang/libc/blob/master/appveyor.yml)
 install:
+  - ps: >-
+        If ($Env:TARGET -eq 'x86_64-pc-windows-gnu') {
+           $Env:PATH += ';C:\msys64\mingw64\bin'
+        } ElseIf ($Env:TARGET -eq 'i686-pc-windows-gnu') {
+           $Env:PATH += ';C:\MinGW\bin'
+        }
   - curl -sSf -o rustup-init.exe https://win.rustup.rs
   - rustup-init.exe --default-host %TARGET% --default-toolchain %CHANNEL% -y
   - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin