Fix build for gcc 15

[jacobmealey]

This patch comes from the downstream debian and was authored by Andreas Beckmann anbe@debian.org here:

https://salsa.debian.org/debian/rc/-/blob/main/debian/patches/gcc-15.patch?ref_type=heads

Fixes #113

[rakitzis]

Hi Jacob, thanks for this patch. It looks good but the sense of the #ifdef means that for environments where __STDC_VERSION__ is undefined (ancient C compilers?) the #else clause will not be active. Could you reverse the sense of the conditional? This way backward compatibility is ensured. Thanks.

(From Google: __STDC_VERSION__ appeared in C94. I'm curious to know whether rc still compiles with C89 but I have no reason to expect it doesn't.)

[jacobmealey]

Hello, that was a good catch. sorry for the spamming of force pushes. I thought I found a wierd case that required extra preprocessor logic, but then i realised it was reduntent.

Also i've compiled this with c89 and c99 and it seems happy, but that was just with gcc.

[rakitzis]

Thanks!