[boost] Building Boost on Haiku

Hi, New booster here. I am building Boost 1.55 on Haiku X86 and have a build issue: -pthreads is not recognized by GCC 4.8 on Haiku X86 and gives an error. pthreads are supported by Haiku by default, so I need to suppress the compiler option but enable pthreads in the build. I used ./bootstrap.sh -PREFIX to set the default lib path as /usr is not used in Haiku Then I started the build with ./b2 install Where and how can I make this config change? I have little jam experience. I also need to suppress use of MPI as this is not ported yet. Many thanks!