Set MSVC_RUNTIME only if it is not set

This allows parent CMake to set it if they want but still
provides standalone users a reasonable default (static)

CMakeLists.txt

@@ -26,7 +26,12 @@ cmake_minimum_required(VERSION 3.21)
 cmake_policy(SET CMP0091 NEW)
 set(CMAKE_OSX_DEPLOYMENT_TARGET 10.13 CACHE STRING "Minimum macOS version")
 set(CMAKE_OSX_ARCHITECTURES "x86_64;arm64" CACHE STRING "Build Universal Always")
-set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>") # Statically link the MSVC Runtime
+if (NOT DEFINED CMAKE_MSVC_RUNTIME_LIBRARY)
+  if (WIN32)
+    message(STATUS "CMAKE_MSVC_RUNTIME_LIBRARY not defined. Setting to static link")
+  endif()
+  set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>") # Statically link the MSVC Runtime
+endif()
 set(CMAKE_COLOR_DIAGNOSTICS ON)
 
 # If your clap supports note expressions you *can* implement the wrapper extension here or you