AppVeyor: add WinSSL builds

Use the oldest and latest Windows SDKs for them.
Also, remove all but one OpenSSL build.

Closes curl#3104

appveyor.yml

@@ -4,49 +4,52 @@ environment:
     matrix:
       - PRJ_GEN: "Visual Studio 9 2008"
         PRJ_CFG: Release
-        OPENSSL: ON
-        TESTING: OFF
-        SHARED: ON
-      - PRJ_GEN: "Visual Studio 11 2012 Win64"
-        PRJ_CFG: Release
-        OPENSSL: ON
+        OPENSSL: OFF
+        WINSSL: ON
         TESTING: OFF
         SHARED: ON
-      - PRJ_GEN: "Visual Studio 12 2013 Win64"
+      - PRJ_GEN: "Visual Studio 14 2015 Win64"
         PRJ_CFG: Release
-        OPENSSL: ON
+        OPENSSL: OFF
+        WINSSL: ON
         TESTING: OFF
         SHARED: ON
       - PRJ_GEN: "Visual Studio 14 2015 Win64"
         PRJ_CFG: Release
         OPENSSL: ON
+        WINSSL: OFF
         TESTING: OFF
         SHARED: ON
       - PRJ_GEN: "Visual Studio 10 2010 Win64"
         PRJ_CFG: Release
         OPENSSL: OFF
+        WINSSL: OFF
         TESTING: ON
         SHARED: OFF
       - PRJ_GEN: "Visual Studio 11 2012 Win64"
         PRJ_CFG: Release
         OPENSSL: OFF
+        WINSSL: OFF
         TESTING: ON
         SHARED: OFF
       - PRJ_GEN: "Visual Studio 12 2013 Win64"
         PRJ_CFG: Release
         OPENSSL: OFF
+        WINSSL: OFF
         TESTING: ON
         SHARED: OFF
       - PRJ_GEN: "Visual Studio 14 2015 Win64"
         PRJ_CFG: Release
         OPENSSL: OFF
+        WINSSL: OFF
         TESTING: ON
         SHARED: OFF
 
 build_script:
     - cmake .
       -G"%PRJ_GEN%"
       -DCMAKE_USE_OPENSSL=%OPENSSL%
+      -DCMAKE_USE_WINSSL=%WINSSL%
       -DBUILD_SHARED_LIBS=%SHARED%
       -DBUILD_TESTING=%TESTING%
       -DCURL_WERROR=ON