Port intmax macro to strict C11

Port intmax macro to strict C11

Needed for Apple clang version 14.0.0 (clang-1400.0.29.202).
* src/common.h (intmax): Define macro only if _Generic works, and
use _Generic in it rather that using EXPR_SIGNED.  This is needed
to make the first argument of verify_expr an integer constant
expression, which is required for strict C11.  Although GCC is
smart enough to treat (1 ? 0 : V) as an integer constant
expression even if V is an integer variable, C11 does not require
support for this sort of thing.