Fix compile error when use msvc 2019 (#754)

Yu Zeng · MaskRay · commit aca02d209b39 · 2021-01-18T19:05:28.000-08:00
See https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/
diff --git a/CMakeLists.txt b/CMakeLists.txt
@@ -45,6 +45,7 @@ if(MSVC)
     /wd4800
     /wd4068 # Disable unknown pragma warning
     /std:c++17
+    /Zc:__cplusplus
     $<$<CONFIG:Debug>:/FS>
   )
   # relink system libs

Original file line number	Diff line number	Diff line change
`@@ -45,6 +45,7 @@ if(MSVC)`
`45`	`45`	`/wd4800`
`46`	`46`	`/wd4068 # Disable unknown pragma warning`
`47`	`47`	`/std:c++17`
	`48`	`+ /Zc:__cplusplus`
`48`	`49`	`$<$<CONFIG:Debug>:/FS>`
`49`	`50`	`)`
`50`	`51`	`# relink system libs`