build: allow CMAKE_MSVC_RUNTIME_LIBRARY #1958 (#1960)

CMakeLists.txt

@@ -122,22 +122,25 @@ if(MSVC)
     add_compile_options($<$<COMPILE_LANGUAGE:C>:/wd4267>)
 
     # handle msvcrt setting being passed in CMAKE_C_FLAGS
-    if(DEFINED CMAKE_MSVC_RUNTIME_LIBRARY)
-        # do not support other methods of setting this (it would be more conformant, tho)
-        message(FATAL_ERROR "please set msvcrt via CMAKE_C_FLAGS")
-    endif()
-    if(CMAKE_C_FLAGS MATCHES "[/-]MTd")
-        set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreadedDebug")
-    elseif(CMAKE_C_FLAGS MATCHES "[/-]MDd")
-        set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreadedDebugDLL")
-    elseif(CMAKE_C_FLAGS MATCHES "[/-]MT")
-        set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded")
-    elseif(CMAKE_C_FLAGS MATCHES "[/-]MD")
-        set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreadedDLL")
-    endif()
+    if(CMAKE_C_FLAGS MATCHES "[/-]M[TD]d?")
+        # ensure CMAKE_MSVC_RUNTIME_LIBRARY is not already defined
+        if(DEFINED CMAKE_MSVC_RUNTIME_LIBRARY)
+            message(FATAL_ERROR "please set the runtime library via either CMAKE_C_FLAGS or CMAKE_MSVC_RUNTIME_LIBRARY, not both")
+        endif()
 
-    # prevent the arg from occurring more than once (not a big deal, just to keep tidy)
-    string(REGEX REPLACE "[/-]M[TD]d?" "" CMAKE_C_FLAGS ${CMAKE_C_FLAGS})
+        if(CMAKE_C_FLAGS MATCHES "[/-]MTd")
+            set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreadedDebug")
+        elseif(CMAKE_C_FLAGS MATCHES "[/-]MDd")
+            set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreadedDebugDLL")
+        elseif(CMAKE_C_FLAGS MATCHES "[/-]MT")
+            set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded")
+        elseif(CMAKE_C_FLAGS MATCHES "[/-]MD")
+            set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreadedDLL")
+        endif()
+
+        # prevent the arg from occurring more than once (not a big deal, just to keep tidy)
+        string(REGEX REPLACE "[/-]M[TD]d?" "" CMAKE_C_FLAGS ${CMAKE_C_FLAGS})
+    endif()
 else()
     if(MINGW)
         execute_process(COMMAND ${CMAKE_C_COMPILER} -dumpmachine