Mingw support update

When trying to build OSRM under MSYS2 with its package Archlinux-ported package system, I have noticed new MinGW incompatibilities. Currently M_1_PI and so on are used in the code but only M_PI defined by CMake, so _USE_MATH_DEFINES could help.  
gcc-ar is also unavailable in gcc 4.9.1 from MSYS2 , hope it will  be ok to disable that fragment.

openmp problem still remained (only manual adding ``gomp`` to osrm-extract helped), but this is to be fixed in stxxl, I guess (compiled with USE_GNU_PARALLEL but still uses OpenMP too).