build: Update *.m4 and configure.ac.
* configure.ac (AC_HEADER_STDC): Remove obsolete feature. (AM_PATH_GTK_3_0): Use PKG_CHECK_MODULES. * m4/pkg.m4: New. * m4/gpg-error.m4: Update from libgpg-error master. * m4/gpgme.m4: Update from gpgme master. * m4/libassuan.m4: Update from libassuan master.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>