Update USE_MSVC_RUNTIME_LIBRARY_DLL for CMake 3.15

This makes USE_MSVC_RUNTIME_LIBRARY_DLL update the directory scope
CMAKE_MSVC_RUNTIME_LIBRARY variable instead of CMAKE_C_FLAGS on CMake
3.15 and later.

Solution proposed by @moritz-h.

Fixes #1783.
Closes #1796.

(cherry picked from commit f4a7329604)

CMakeLists.txt

@@ -80,8 +80,10 @@ if (MSVC)
         # Workaround for VS 2008 not shipping with stdint.h
         list(APPEND glfw_INCLUDE_DIRS "${GLFW_SOURCE_DIR}/deps/vs2008")
     endif()
+endif()
 
-    if (NOT USE_MSVC_RUNTIME_LIBRARY_DLL)
+if (MSVC AND NOT USE_MSVC_RUNTIME_LIBRARY_DLL)
+    if (${CMAKE_VERSION} VERSION_LESS 3.15)
         foreach (flag CMAKE_C_FLAGS
                       CMAKE_C_FLAGS_DEBUG
                       CMAKE_C_FLAGS_RELEASE
@@ -96,6 +98,8 @@ if (MSVC)
             endif()
 
         endforeach()
+    else()
+        set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")
     endif()
 endif()