How can Autoconf help with the transition to stricter compilation defaults?

[Bruno Haible via cfe-commits]

Paul Eggert wrote:
> > AC_CHECK_FUNC *should not*  just probe for linkability of a symbol
> 
> ... Autoconf cannot 
> be expected to know every signature of every function in every library.

Clang will surely not acquire knowledge about "every library", right,
only about the C library according to relevant standards (ISO C, POSIX)?

> ... In the meantime if 
> Clang becomes pickier by default it would be helpful if there were a 
> well-defined way to shut off Clang's pickiness.

To me it seems the problem is not Clang's pickiness, but rather the
assumptions that it makes about the target environment. There is a
documented way to ask Clang and GCC "assume a free-standing implementation,
not an environment that has the ISO C / POSIX / LSB / ... functions":
  1) -ffreestanding         [1][2]
  2) -fno-builtin

Can we assume that these options will continue to work?

If so, all Autoconf needs to do is to pass these options to the compiler
in AC_CHECK_FUNC checks. Then we don't need to add knowledge about each
of the hundreds of standard libc functions into Autoconf.

Bruno

[1] https://clang.llvm.org/docs/ClangCommandLineReference.html
[2] https://gcc.gnu.org/onlinedocs/gcc-12.2.0/gcc/C-Dialect-Options.html