boost: Update 4

boost: Update 4

  Major Updates
  - Added support for Python 3.5.
  - Removed the restriction for the target MPC85xx when using uclibc [1].
    - No longer required since uclibc was removed from trunk.
  - Added option to force static compilation.
  - Added option to force linking statically to the C++ standard library and compiler runtime support libraries.
  - Added option to disable multithreading support. It can be helpfull for those who wish to fully optimise their code.
    - Some boost libraries will require multithreading to be active. For those, this option is active as a requirement.

  Minor Updates
  - Added -fPIC to CFLags [2].
    - python requires independent position code when statically compiling.

  References:
  [1] - https://github.com/openwrt/packages/issues/1621
  [2] - https://github.com/openwrt/packages/issues/1938

Signed-off-by: Carlos M. Ferreira <carlosmf.pt@gmail.com>