hdf5_der

[svn-r22035] Added configure link test to see if we can really link with the real 
time library.

This change is necessary, as aio calls result in link failures when we 
link with the -static flag.  The new test in configure.in checks to see
if we think we have POSIX AIO, and if we do, it does a AC_TRY_LINK with
one of the aio calls.  Note that to do this, the test must make temporary
changes to LIBS and (if the STATIC_EXEC flag is set) to LDFLAGS.

For now, if the link test fails, we kill AIO completely.  We will have
to revist this if we ever support anything other than POSIX AIO.

Tested on jam (serial, with and without the --enable-static-exec configure
flag, and also parallel), and Phoenix (64 bit linux, serial only).

In testing, I noticed one un-related issue that I didn't have time to
chase.  It seems that "-lrt" is showing up twice in the list of extra
libraries list in the configure results summary.  This doesn't seem to 
be causing problems, but it should be looked into.