* configure.ac: Use AC_CONFIG_HEADERS instead of AM_CONFIG_HEADER.
Use AC_USE_SYSTEM_EXTENSIONS instead of AC_GNU_SOURCE.
Use AS_HELP_STRING instead of AC_HELP_STRING.
(AC_ISC_POSIX): Replace by AC_SEARCH_LIBS.
(AC_STDC_HEADERS): Replace by AC_HEADER_STDC.
(byte, ushort, ulong, u16, u32): Use AC_CHECK_TYPES.
* m4/check_zlib.m4: Use AS_HELP_STRING instead of AC_HELP_STRING.
Use AC_MSG_ERROR instead of AC_ERROR.
* m4/gettext.m4: Update from gnulib.
* m4/gpg-error.m4: Update from libgpg-error.
* m4/gpgme.m4: Update from gpgme.
* m4/libassuan.m4: Update from libassuan.