Configure script updates
Fixes some complaints about obsolete autoconf symbols. We still get:
configure.ac:56: warning: The macro `AC_PROG_CC_C99' is obsolete.
Using AC_PROG_CC only is meant to replace this but removing the
AC_PROG_CC_C99 breaks the build on RHEL7 so it's going to stay for now.
Signed-off-by: Andrew Price <anprice@redhat.com>