Add a wiki page about which C standard is used by SDL.

[icculus]

An entry in the Wiki about which C standard is used by SDL could be useful.
Something like:

### C Standard:
Building a project with SDL: C89 and newer
Building the SDL library itself: C99 and newer

And maybe a note about which C99 features are used to build SDL itself:

### C99 features used to build the SDL library itself:
// comments
variable declarations in the for loop init-clause
va_copy function from <stdarg.h> (src/SDL_log.c)
inline function specifier (in dbus-1.0 headers)

Originally posted by @Sackzement in libsdl-org/SDL#9440 (comment)