Add support for settable VS_INSTALLATION_PATH - enabling eWDK (#74)

* Added support for the EWDK

You can now use the EWDK to build with this toolchain instead of a pre-installed VS version.

* Attempt to get github actions environment working

* Added *~ pattern to .gitignore for temporary editor files

* Don't set CMAKE_WINDOWS_KITS_10_DIR from environment

This avoids situations where WindowsSdkDir is set in the environment but we want to select a specific version via the pre-existing mechanisms in thie toolchain.
It's better to require EWDK users to explicitly state what they want by setting CMAKE_WINDOWS_KITS_10_DIR via cmake cache vars on the commandline.

.gitignore

@@ -1,3 +1,4 @@
 __*
 .vscode
-.vs
+.vs
+*~

Windows.Kits.cmake

@@ -56,6 +56,9 @@ endif()
 
 if(NOT CMAKE_WINDOWS_KITS_10_DIR)
     get_filename_component(CMAKE_WINDOWS_KITS_10_DIR "[HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\Microsoft SDKs\\Windows\\v10.0;InstallationFolder]" ABSOLUTE CACHE)
+    if ("${CMAKE_WINDOWS_KITS_10_DIR}" STREQUAL "/registry")
+    	unset(CMAKE_WINDOWS_KITS_10_DIR)
+    endif()
 endif()
 
 if(NOT CMAKE_WINDOWS_KITS_10_DIR)

Windows.MSVC.toolchain.cmake

@@ -37,7 +37,8 @@
 # | CMAKE_VS_PLATFORM_TOOLSET_VERSION           | The version of the MSVC toolset to use. For example, 14.29.30133. Defaults to the highest available.                     |
 # | CMAKE_VS_PLATFORM_TOOLSET_HOST_ARCHITECTURE | The architecture of the toolset to use. Defaults to 'x64'.                                                               |
 # | CMAKE_WINDOWS_KITS_10_DIR                   | The location of the root of the Windows Kits 10 directory.                                                               |
-# | VS_EXPERIMENTAL_MODULE                      | Whether experimental module support should be enabled.
+# | VS_INSTALLATION_PATH                        | The location of the root of the Visual Studio installation. If not specified VSWhere will be used to search for one.     |
+# | VS_EXPERIMENTAL_MODULE                      | Whether experimental module support should be enabled.                                                                   |
 # | VS_USE_SPECTRE_MITIGATION_RUNTIME           | Whether the compiler should link with a runtime that uses 'Spectre' mitigations. Defaults to 'OFF'.                      |
 #
 # The toolchain file will set the following variables:
@@ -108,14 +109,16 @@ endif()
 
 # Find Visual Studio
 #
-findVisualStudio(
-    VERSION ${CMAKE_VS_VERSION_RANGE}
-    PRERELEASE ${CMAKE_VS_VERSION_PRERELEASE}
-    PRODUCTS ${CMAKE_VS_PRODUCTS}
-    PROPERTIES
-        installationVersion VS_INSTALLATION_VERSION
-        installationPath VS_INSTALLATION_PATH
-)
+if(NOT VS_INSTALLATION_PATH)
+	findVisualStudio(
+		VERSION ${CMAKE_VS_VERSION_RANGE}
+		PRERELEASE ${CMAKE_VS_VERSION_PRERELEASE}
+		PRODUCTS ${CMAKE_VS_PRODUCTS}
+		PROPERTIES
+			installationVersion VS_INSTALLATION_VERSION
+			installationPath VS_INSTALLATION_PATH
+	)
+endif()
 
 message(VERBOSE "VS_INSTALLATION_VERSION = ${VS_INSTALLATION_VERSION}")
 message(VERBOSE "VS_INSTALLATION_PATH = ${VS_INSTALLATION_PATH}")