use the APR feature test macro to check for sys/uio.h; it is the

only one defined and it is used in other Apache code;
this gets mod_rewrite to compile on RedHat 6.0 (and perhaps some
other systems)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87088 13f79535-47bb-0310-9956-ffa450edef68