Invalid warning option on MSVC

[phprus]

File: https://github.com/gulrak/filesystem/blob/master/cmake/GhcHelper.cmake#L53

Invalid option: /wd"4996" (error: cl : Command line error D8021 : invalid numeric argument '/wd"4996"')

Correct option: /wd4996 (MSDN: https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-3-c4996?view=msvc-160)

[gulrak]

Strangely I got no error with both syntaxes on my VS2019 installation or on AppVeyor with 2015/2017/2019 runs, but I changed it anyway. Thanks for reporting it.

[gulrak]

This is now part of release v1.5.2