Feat: setup vc environment during configuration

This would allow us to build using ninja from the command line.
This is currently not possible as CMake cannot find `cl` when building from a "normal" shell.
The vcvarsall script would have to be called before.
This is very inconvenient.
With this change the vc environment would be setup during configuration.
However, as discussed [here](https://github.com/MarkSchofield/WindowsToolchain/issues/58) this does not work correctly yet.

src/Index.cmake

@@ -48,3 +48,18 @@ macro (find_package)
 	# 	message (FATAL_ERROR "Package ${ARGV0} found as a Target; skip find_package")
 	# endif ()
 endmacro ()
+
+CPMAddPackage (
+	NAME project_options
+	GITHUB_REPOSITORY aminya/project_options
+	VERSION 0.36.4
+	DOWNLOAD_ONLY YES
+)
+
+include (${project_options_SOURCE_DIR}/src/VCEnvironment.cmake)
+
+if ("${CMAKE_TOOLCHAIN_FILE}" STREQUAL "")
+	msvc_toolchain ()
+else ()
+	message (STATUS "project_options: skipping msvc_toolchain as CMAKE_TOOLCHAIN_FILE is set")
+endif ()