tests: fix uio building w/out preadv/pwritev

The preadv/pwritev symbols weren't added to glibc until the 2.10 release,
so trying to build the uio test leads to link failures.  Add configure
tests and update uio.test to handle this.

* configure.ac (AC_CHECK_FUNCS): Add preadv/pwritev.
* tests/uio.c: Include config.h.
(main): Check for HAVE_PREADV and HAVE_PWRITEV.
* tests/uio.test: Check exit status of uio helper.
3 files changed