build: detect overflow for [re]allocate_array.

build: detect overflow for [re]allocate_array.

Use reallocarray() when we have it (i.e. in OpenBSD system). When we
don't, use equivalent overflow detection for our allocate_array and
reallocate_array functions.

Remove lib/reallocarray.c from our LIBOBJS as we no longer need it.

Provide a fallback SIZE_MAX macro definition in flexint.h (not
preprocessor friendly, but enough for our reallocate_array use case).