configure: Improve C99 compatibility

[fweimer-rh]

C99 compilers do not necessarily recognize implicit ints, so use int explicitly as the return type of main in CAN_DO.

In HAVE_FUNC, supply a custom prototype to avoid an implicit function declaration, otherwise this test fails at compile time as well. The same prototype declaration is used by autoconf, so future compilers are likely to remain compatible.

[wjaguar]

Thanks. And which compiler specifically is that fussy?

[fweimer-rh]

There are plans for Clang 16 to make the switch, and perhaps GCC 14. For now, I'm using an instrumented GCC version to prepare for the transition:

• https://fedoraproject.org/wiki/Toolchain/PortingToModernC#Discovering_Packages_Which_Need_Fixing