m4: Update with newer autoconf constructs.
m4: Update with newer autoconf constructs.

* src/gpgme.m4: Replace AC_HELP_STRING to AS_HELP_STRING.

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

gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme