CMake: Fix for #1699 msvc runtime selection issues

Previously I eschewed the use of CMAKE_MSVC_RUNTIME_LIBRARY in favour of setting the flags using target_compile_options so that they would propagate to consumers.

However, it has been raised that a dependency( independent to godot-cpp ) that doesn't set any runtime flags, which relies purely on the CMAKE_MSVC_RUNTIME_LIBRARY variable will very likely not have the correct msvc runtime flags set.

Where MSVC documentation states "All modules passed to a given invocation of the linker must have been compiled with the same runtime library compiler option (/MD, /MT, /LD)."

It was also mentioned that message type WARNING is not ideal for notifying consumers.

So I have updated the cmake code to use CMAKE_MSVC_RUNTIME_LIBRARY  over target-compile_options. And set it as a CACHE STRING variable so that it can be overridden if desired. We still message consumers, but as a NOTICE.

cmake/windows.cmake

@@ -1,10 +1,16 @@
 #[=======================================================================[.rst:
 Windows
 -------
-
 This file contains functions for options and configuration for targeting the
 Windows platform
 
+Because this file is included into the top level CMakelists.txt before the
+project directive, it means that
+
+* ``CMAKE_CURRENT_SOURCE_DIR`` is the location of godot-cpp's CMakeLists.txt
+* ``CMAKE_SOURCE_DIR`` is the location where any prior ``project(...)``
+    directive was
+
 MSVC Runtime Selection
 ----------------------
 
@@ -19,48 +25,50 @@ Default: ``CMAKE_MSVC_RUNTIME_LIBRARY="MultiThreaded$<$<CONFIG:Debug>:Debug>DLL"
 This initializes each target's ``MSVC_RUNTIME_LIBRARY`` property at the time of
 target creation.
 
+it is stated in the msvc_ documentation that: "All modules passed to a given
+invocation of the linker must have been compiled with the same runtime library
+compiler option (/MD, /MT, /LD)."
+
 This creates a conundrum for us, the ``CMAKE_MSVC_RUNTIME_LIBRARY`` needs to be
 correct at the time the target is created, but we have no control over the
 consumers CMake scripts, and the per-target ``MSVC_RUNTIME_LIBRARY`` property
 is not transient.
 
-We need ``CMAKE_MSVC_RUNTIME_LIBRARY`` to be ``"$<1:>"`` to ensure it
-will not add any flags. And then use ``target_compile_options()`` so that our
-flags will propagate to consumers.
+It has been raised that not using ``CMAKE_MSVC_RUNTIME_LIBRARY`` can also cause
+issues_ when a dependency( independent to godot-cpp ) that doesn't set any
+runtime flags, which relies purely on the ``CMAKE_MSVC_RUNTIME_LIBRARY``
+variable will very likely not have the correct msvc runtime flags set.
+
+So we'll set ``CMAKE_MSVC_RUNTIME_LIBRARY`` as CACHE STRING so that it will be
+available for consumer target definitions, but also be able to be overridden if
+needed.
 
-In the interests of playing nicely we detect whether we are being consumed
-and notify the consumer that we are setting ``CMAKE_MSVC_RUNTIME_LIBRARY``,
-that dependent targets rely on it, and point them to these comments as to why.
+Additionally we message consumers notifying them and pointing to this
+documentation.
 
 .. _CMP0091:https://cmake.org/cmake/help/latest/policy/CMP0091.html
 .. _property:https://cmake.org/cmake/help/latest/variable/CMAKE_MSVC_RUNTIME_LIBRARY.html
 .. https://discourse.cmake.org/t/mt-staticrelease-doesnt-match-value-md-dynamicrelease/5428/4
+.. _msvc: https://learn.microsoft.com/en-us/cpp/build/reference/md-mt-ld-use-run-time-library
+.. _issues: https://github.com/godotengine/godot-cpp/issues/1699
 
 ]=======================================================================]
-if( PROJECT_NAME ) # we are not the top level if this is true
-    if( DEFINED CMAKE_MSVC_RUNTIME_LIBRARY )
-        # Warning that we are clobbering the variable.
-        message( WARNING "setting CMAKE_MSVC_RUNTIME_LIBRARY to \"$<1:>\"")
-    else(  )
-        # Notification that we are setting the variable
-        message( STATUS "setting CMAKE_MSVC_RUNTIME_LIBRARY to \"$<1:>\"")
-    endif()
-endif()
-set( CMAKE_MSVC_RUNTIME_LIBRARY "$<1:>" CACHE INTERNAL "Select the MSVC runtime library for use by compilers targeting the MSVC ABI." )
-
-#[============================[ Windows Options ]============================]
 function( windows_options )
-
     option( GODOT_USE_STATIC_CPP "Link MinGW/MSVC C++ runtime libraries statically" ON )
-
     option( GODOT_DEBUG_CRT "Compile with MSVC's debug CRT (/MDd)" OFF )
 
+    message( STATUS "If not already cached, setting CMAKE_MSVC_RUNTIME_LIBRARY.\n"
+            "\tFor more information please read godot-cpp/cmake/windows.cmake")
+
+    set( CMAKE_MSVC_RUNTIME_LIBRARY
+            "MultiThreaded$<IF:$<BOOL:${GODOT_DEBUG_CRT}>,DebugDLL,$<$<NOT:$<BOOL:${GODOT_USE_STATIC_CPP}>>:DLL>>"
+            CACHE STRING "Select the MSVC runtime library for use by compilers targeting the MSVC ABI.")
 endfunction()
 
+
 #[===========================[ Target Generation ]===========================]
 function( windows_generate )
     set( STATIC_CPP "$<BOOL:${GODOT_USE_STATIC_CPP}>")
-    set( DEBUG_CRT "$<BOOL:${GODOT_DEBUG_CRT}>" )
 
     set_target_properties( ${TARGET_NAME}
             PROPERTIES
@@ -76,11 +84,6 @@ function( windows_generate )
             >
     )
 
-    target_compile_options( ${TARGET_NAME}
-        PUBLIC
-            $<${IS_MSVC}:$<IF:${DEBUG_CRT},/MDd,$<IF:${STATIC_CPP},/MT,/MD>>>
-    )
-
     target_link_options( ${TARGET_NAME}
             PUBLIC