[Core] Add "strict" mode for strict C99 compliance

[FintasticMan]

Depends on #119.

This just allows users of the library to define CLAY_STRICT to get rid of the use of the non-standard ##__VA_ARGS__ construction, if they're OK with making sure that the affected macros aren't used without an argument.

I'm not sure if this is something we want, so I'd be very open to feedback! I like how much simpler the CLAY_SIZING_* macros are without having to worry about making sure that they also work when no arguments are given, so it might not even be out of the picture to just replace the current implementation with this strict one.

[VisenDev]

perhaps

#if defined(CLAY_STRICT) && CLAY_STRICT == 1

would be a better way to enable CLAY_STRICT? That way you can use

#define CLAY_STRICT 0
#define CLAY_STRICT 1

to enable/disable

[FintasticMan]

Superseded by #150.