libretro-common: fix implicit declarations

[atsampson]

strdup and realpath are only declared by glibc's headers if _XOPEN_SOURCE >= 500.

(I checked libretro-common upstream, and this doesn't seem to have been fixed there; maybe they're not building with -std=c99 or similar. Previously it would just have produced a warning but an implicit declaration is a hard error with GCC 14.)

[zeromus]

that broke the mac build. strlcpy this time. increase the version number or something.

[rofl0r]

usually 700 is the lowest value one uses for the XOPEN feature test macro.
strlcpy is a BSD-originated thing, so i guess additional defining _BSD_SOURCE could fix it.

[atsampson]

Looking at macOS's cdefs.h and string.h, it looks like strlcpy is only declared if __DARWIN_C_LEVEL >= 900000, so there's no value of _XOPEN_SOURCE that would enable it. It might need an #ifdef for macOS to define _DARWIN_C_SOURCE there (or just not define _XOPEN_SOURCE there, if the default level was working already).