pnnx static msvc crt option

tools/pnnx/CMakeLists.txt

@@ -2,7 +2,20 @@ project(pnnx)
 cmake_minimum_required(VERSION 3.12)
 
 if(POLICY CMP0074)
-  cmake_policy(SET CMP0074 NEW)
+    cmake_policy(SET CMP0074 NEW)
+endif()
+
+if(NOT CMAKE_VERSION VERSION_LESS "3.15")
+    # enable CMAKE_MSVC_RUNTIME_LIBRARY
+    cmake_policy(SET CMP0091 NEW)
+endif()
+
+if(MSVC AND NOT CMAKE_VERSION VERSION_LESS "3.15")
+    option(PNNX_BUILD_WITH_STATIC_CRT "Enables use of statically linked CRT for statically linked ncnn" OFF)
+    if(PNNX_BUILD_WITH_STATIC_CRT)
+        # cmake before version 3.15 not work
+        set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")
+    endif()
 endif()
 
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")