char8_t: support glibc <2.36

[cwsmith]

This PR addresses a compilation issue with the nvtx 3.3.0 package, with the python bindings enabled (as a dep of py-torch), when built with GCC 15.2.0 and glibc 2.34 on a RHEL9 system. IIUC, glibc 2.36 or newer is needed to use char8_t from the c20 standard (https://lists.gnu.org/archive/html/info-gnu/2022-08/msg00000.html).

I was installing via spack and have a PR for a patch here: spack/spack-packages#4531

Note, I have not directly built/tested these changes outside of spack. Please let me know if there is some testing I can/should do on my end.

On our system, the following code reproduces the bug:

$ cat foo.c 
#include <stdio.h> 
//---- uncomment the following block to compile ----
//#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
//#ifndef __cpp_char8_t
//typedef unsigned char char8_t;
//#endif
//#endif
int main() { char8_t a; printf("%ld\n", (long)__STDC_VERSION__); return 0; }

$ gcc --version
gcc (Spack GCC) 15.2.0
Copyright (C) 2025 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

##  show glibc version
$ ldd --version   
ldd (GNU libc) 2.34
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Written by Roland McGrath and Ulrich Drepper.

$ gcc foo.c -o foo
foo.c: In function 'main':
foo.c:9:14: error: unknown type name 'char8_t'; did you mean 'char'?
    9 | int main() { char8_t a; printf("%ld\n", (long)__STDC_VERSION__); return 0; }
      |              ^~~~~~~
      |              char

[evanramos-nvidia]

@cwsmith Thanks for the report and for the proposal! I believe there is a better way to approach this fix. Could you test my branch char8_t to see if it resolves the build failure on your system? https://github.com/NVIDIA/NVTX/tree/char8_t