zig.h: Use C99/C11 headers if available regardless of language standard.
If present, these headers are usable even when compiling for older C language
versions.
Most notably, this enables zig.h atomics to work with slimcc and TinyCC in C99
mode (and earlier).
0e934c7df1Alex Rønne Petersen committed on 12/3/2024, 11:07:34 AMparent1c43c2c