Nothing checks for the STDC_HEADERS flag this set, and all supported
systems have C89 compatible headers now.
Clears autoconf warnings of:
configure.ac:93: warning: The macro `AC_HEADER_STDC' is obsolete.
configure.ac:93: You should run autoupdate.
./lib/autoconf/headers.m4:704: AC_HEADER_STDC is expanded from...
configure.ac:93: the top level