fix pkg-config --static when pthread is reported

 - refs https://github.com/DennisOSRM/node-osrm/pull/28
 - handles case where `$Boost_LIBRARIES` looks like `/usr/lib/libboost_filesystem-mt.so;/usr/lib/libboost_iostreams-mt.so;/usr/lib/libboost_program_options-mt.so;/usr/lib/libboost_regex-mt.so;/usr/lib/libboost_system-mt.so;/usr/lib/libboost_thread-mt.so;pthread`