Put the right MinGW directory into the path.

It looks as if the 32-bit MinGW directory is in the path by default;
remove it and then put the appropriate directory, whether it's the
32-bit version or some 64-bit version, into the path.

Maybe this fixes the "gcc can't compile a simple program" error we're
getting.

.appveyor.yml

@@ -43,9 +43,19 @@ environment:
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
       GENERATOR: "MinGW Makefiles"
       SDK: WpdPack
+      MINGW_PATH: C:\MinGW\bin
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+      GENERATOR: "MinGW Makefiles"
+      SDK: npcap-sdk-0.1
+      MINGW_PATH: C:\MinGW\bin
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
       GENERATOR: "MinGW Makefiles"
       SDK: WpdPack
+      MINGW_PATH: C:\mingw-w64\x86_64-8.1.0-posix-seh-rt_v6-rev0
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+      GENERATOR: "MinGW Makefiles"
+      SDK: npcap-sdk-0.1
+      MINGW_PATH: C:\mingw-w64\x86_64-8.1.0-posix-seh-rt_v6-rev0
 
 build_script:
   #
@@ -54,8 +64,13 @@ build_script:
   - type NUL >.devel
   - md build
   - cd build
-  - if "%GENERATOR%"=="MinGW Makefiles" set PATH=C:\mingw-w64\x86_64-8.1.0-posix-seh-rt_v6-rev0;%PATH%
-  # Workaround for CMake not wanting sh.exe on PATH for MinGW
+  # Remove the default MinGW path
+  - if "%GENERATOR%"=="MinGW Makefiles" set PATH=%PATH:C:\MinGW\bin;=%
+  # Add the specified MinGW path
+  - if "%GENERATOR%"=="MinGW Makefiles" set PATH=%MINGW_PATH%;%PATH%
+  # Remove the path to Git, so that we don't pick up its sh.exe, as
+  # that breaks MinGW builds - CMake checks for that and fails in the
+  # configuration stage
   - if "%GENERATOR%"=="MinGW Makefiles" set PATH=%PATH:C:\Program Files\Git\usr\bin;=%
   - cmake -DPacket_ROOT=c:\projects\libpcap\Win32\%SDK% -G"%GENERATOR%" ..
   - if NOT "%GENERATOR%"=="MinGW Makefiles" msbuild /m /nologo /p:Configuration=Release pcap.sln