dlopen: only FI_WARN if user specified $FI_EXTDIR

Specifically: don't emit a warning message if the default/compiled-in
extdir does not exist.

Also remove a debugging FI_WARN for when we detect static linking.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
Merge pull request #195 from jsquyres/topic/dlopen-fixes

dlopen: sanity check the DL directory