[LTP] [PATCH] config: Explicitly set gnu99

[Petr Vorel]

Hi Richie, Cyril,

...
> >> > I woder if this would work with different compilers. I suppose clang
> >> > does work, but what about icc?

> >> Clang is fine. I doubt that icc compiles LTP at present unless it does
> >> support GNU extensions.

> > Fair enough.

> >> > I supose that we get tons of 'can't do pointer arithmetics on void*
> >> > pointers and stuff' with plain c99. Maybe it would be better to clean
> >> > these out instead.

FYI gnu99 works on our CI
https://github.com/pevik/ltp/actions/runs/3521626082

and c99 not
https://github.com/pevik/ltp/actions/runs/3521624383
parse_opts.c:458:35: error: implicit declaration of function 'sbrk' [-Werror=implicit-function-declaration]
parse_opts.c:475:27: error: 'CLOCK_MONOTONIC' undeclared (first use in this function)
parse_opts.c:208:47: error: 'optarg' undeclared (first use in this function)
and many other errors

Richie probably found that himself.

So that I'm for merging this and Jan's fixes on the top of it.
I suppose there is no feature dependency on autotools (i.e. we don't need to add
-std=gnu99 to any of macros in m4/.

Kind regards,
Petr