CMake: Force enable MSVC multi-processor build (/MP)

CMakeLists.txt

@@ -44,11 +44,14 @@ endif()
 
 project(ncnn)
 
-if(MSVC AND NOT CMAKE_VERSION VERSION_LESS "3.15")
-    option(NCNN_BUILD_WITH_STATIC_CRT "Enables use of statically linked CRT for statically linked ncnn" OFF)
-    if(NCNN_BUILD_WITH_STATIC_CRT)
-        # cmake before version 3.15 not work
-        set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")
+if(MSVC)
+    add_compile_options(/MP)
+    if(NOT CMAKE_VERSION VERSION_LESS "3.15")
+        option(NCNN_BUILD_WITH_STATIC_CRT "Enables use of statically linked CRT for statically linked ncnn" OFF)
+        if(NCNN_BUILD_WITH_STATIC_CRT)
+            # cmake before version 3.15 not work
+            set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")
+        endif()
     endif()
 endif()