libmusicbrainz: Add -lpthread option in LDFLAGS.

* CodeSourcery toolchain 2013.05-29 gives errors about
pthread symbols. The toolchain is not able to load
libpthread.so automatically so we have to link it manually.

Signed-off-by: Mikhail Durnev <mikhail_durnev@mentor.com>
Merge pull request #5 from mdurnev/mdurnev/musicbrainz

libmusicbrainz: Add -lpthread option in LDFLAGS.