autoconf >= 2.70 produces the following warnings in current configure.ac:
configure.ac:51: warning: The macro `AC_HEADER_STDC' is obsolete.
configure.ac:51: You should run autoupdate.
./lib/autoconf/headers.m4:704: AC_HEADER_STDC is expanded from...
configure.ac:51: the top level
configure.ac:75: warning: AC_OUTPUT should be used without arguments.
configure.ac:75: You should run autoupdate.
autoconf >= 2.70 produces the following warnings in current configure.ac:
This PR fixes those warnings.