Use WinSSPI instead of OpenSSL

Appveyor has OpenSSL installed, so cURL tries to link against it.
This causes dll not found errors because we would also need to ship
OpenSSL, so we link against Windows SSPI instead.

appveyor.yml

@@ -22,7 +22,7 @@ install:
 before_build:
   - mkdir build
   - cd build
-  - cmake -G "Visual Studio 15 2017 Win64" -DCITRA_USE_BUNDLED_QT=1 -DCITRA_USE_BUNDLED_SDL2=1 ..
+  - cmake -G "Visual Studio 15 2017 Win64" -DCITRA_USE_BUNDLED_QT=1 -DCITRA_USE_BUNDLED_SDL2=1 -DCMAKE_USE_OPENSSL=0 ..
   - cd ..
 
 build: