cmake: update vcpkg mod for appveyor build cache

On appveyor we only cache the `vcpkg/installed` directory, so vcpkg
itself still needs to be pulled from git.

Also update vcpkg from git if it has already been cloned, and always
rebuild vcpkg itself.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

cmake/Set-Toolchain-vcpkg.cmake

@@ -16,25 +16,55 @@ if(VCPKG_TARGET_TRIPLET)
         )
 
         if(NOT git_status EQUAL 0)
-            message(FATAL_ERROR "Error cloning vcpkg, please make sure git for windows is installed correctly, it can be installed from Visual Studio components")
+            message(FATAL_ERROR "Error cloning vcpkg from git, please make sure git for windows is installed correctly, it can be installed from Visual Studio components")
+        endif()
+    else()
+        # this is the case when we cache vcpkg/installed with the appveyor build cache
+        if(NOT EXISTS ${VCPKG_ROOT}/.git)
+            set(git_commands
+                "git init"
+                "git remote add origin https://github.com/microsoft/vcpkg.git"
+                "git fetch --all --prune"
+                "git reset --hard origin/master"
+                "git branch --set-upstream-to=origin/master master"
+            )
+            foreach(git_command ${git_commands})
+                separate_arguments(git_command)
+
+                execute_process(
+                    COMMAND ${git_command}
+                    RESULT_VARIABLE git_status
+                    WORKING_DIRECTORY ${VCPKG_ROOT}
+                )
+                
+                if(NOT git_status EQUAL 0)
+                    break()
+                endif()
+            endforeach()
+        else()
+            execute_process(
+                COMMAND git pull --rebase
+                RESULT_VARIABLE git_status
+                WORKING_DIRECTORY ${VCPKG_ROOT}
+            )
+        endif()
+
+        if(NOT git_status EQUAL 0)
+            message(FATAL_ERROR "Error updating vcpkg from git, please make sure git for windows is installed correctly, it can be installed from Visual Studio components")
         endif()
     endif()
 
-    # build vcpkg if not built
+    # build latest vcpkg
     if(WIN32)
-        if(NOT EXISTS ${VCPKG_ROOT}/vcpkg.exe)
-            execute_process(
-                COMMAND bootstrap-vcpkg.bat
-                WORKING_DIRECTORY ${VCPKG_ROOT}
-            )
-        endif()
+        execute_process(
+            COMMAND bootstrap-vcpkg.bat
+            WORKING_DIRECTORY ${VCPKG_ROOT}
+        )
     else()
-        if(NOT EXISTS ${VCPKG_ROOT}/vcpkg)
-            execute_process(
-                COMMAND ./bootstrap-vcpkg.sh
-                WORKING_DIRECTORY ${VCPKG_ROOT}
-            )
-        endif()
+        execute_process(
+            COMMAND ./bootstrap-vcpkg.sh
+            WORKING_DIRECTORY ${VCPKG_ROOT}
+        )
     endif()
 
     foreach(pkg ${VCPKG_DEPS})