libosip2: disable multi-thread support for toolchains without threads

Also added license information

Fixes:
http://autobuild.buildroot.org/results/dcf/dcfbdc1a1ee032881ad5e2f7ca6622d90fc7cb19



[Peter: License is LGPLv2.1+
Signed-off-by: Gilles Talis <gilles.talis@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>