_GNU_SOURCE to compile with llvm/clang

[OH-AU]

To successfully compile with clang 18, I needed to add:
#define _GNU_SOURCE
to ONElib.c
No other change required. There may of course be a better way to address the issue?
Error generated without was:

ONElib.c:1792:29: error: call to undeclared function 'vasprintf'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
1792 | va_start (args, format) ; vasprintf (&p.command, format, args) ; va_end (args) ;