config: don't force shared linkage on FreeBSD

-shared was hardcoded, so when building with --disable-shared it amounts
to trying to do shared linkage against static libs, which naturally
fails.

The fix is straightforward; just don't hardcode it. libtool will work
out what to do.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16427