GZIP, M_PI & point.h

- Changed all occurrences of RDKit::PI into M_PI
- added #ifdef M_PI (...) #endif in all relevant places
- made length() and sqLength() method consistent
  with respect to usage of pow(x, 2) vs x*x in
  Code/Geometry/point.h
- removed gzip-related boost.iostreams dependency and
  replaced with portable "cmake -E tar xzf" command
  in Code/ForceField/MMFF/CMakeLists.txt