tools/gnulib: add macros to skip reallocarray() functions

tools/gnulib: add macros to skip reallocarray() functions

For modules that depend on the reallocarray module, like ialloc, xalloc,
and safe-alloc, it was not possible to skip importing the reallocarray
module as they all contained at least one function that called
reallocarray() and would cause build failure if the host system didn't
declare it.

This upstreamable patch adds macros that toggle whether to define
functions that depend on reallocarray() based on whether the reallocarray
module is being imported.

Tested-by: Georgi Valkov <gvalkov@gmail.com> # MacOS
Signed-off-by: Michael Pratt <mcpratt@pm.me>
Signed-off-by: Tony Ambardar <itugrok@yahoo.com>