build: Add -Wpointer-arith if available.
build: Add -Wpointer-arith if available.

* configure.ac: Add -Wpointer-arith if available.

--

Signed-off-by: NIIBE Yutaka <34feb69f206f6fec69fb4bd79c126fcbde2c8e6a@fsij.org>

gpg/npth,gpg/npth