configure.ac: require pthread-stubs only where available

configure.ac: require pthread-stubs only where available

The project is a thing only for BSD platforms. Or in other words - for
any other platforms building/installing pthread-stubs results only in a
pthread-stub.pc file.

And even where it provides a DSO, there's a fundamental design issue
with it - see the pthread-stubs mailing list for the specifics.

v2: Update comment above the switch statement (Jon Turney).

Reviewed-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Acked-by: Gary Wong <gtw@gnu.org>
Tested-by: Eric Engestrom <eric.engestrom@imgtec.com>
Acked-by: Randy Fishel <randy.fishel@oracle.com>
Cc: Niveditha Rau <niveditha.rau@oracle.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>