Fix build error caused by use of deprecated POSIX name of strdup

[Nytra]

Changes strdup to _strdup

Fixes the following build error on Windows 11 with VS2022:

rtmidi_c.cpp(420,19): error C4996: 'strdup': The POSIX name for this item is deprecated. Instead, use the ISO C and C++ conformant name: _strdup. See online help for details.

[Nytra]

Looks like this breaks the build on Linux and MacOS

[Nytra]

Could be fixed with the following I think, although it's a bit of a hack

#ifdef _MSC_VER
#define strdup _strdup
#endif

[Nytra]

Changing SDLCheck to false in rtmidilib.vcxproj fixes it too, not sure if that would be preferable

[keinstein]

Ironically C24 has adopted the strdup functiion as mentioned in the ANSI C Blog so _strdup will probably be deprecated in the far future.