Obtain char* from boost::filesystem::path on all systems

The definition of c_str function in boost is
``const value_type*   c_str() const ``
and on some compilers and runtimes in can be not char* (wchar_t* on Windows, for example), To guarantee getting char*, it is recommended to use ``.string().c_str()``
http://stackoverflow.com/questions/11352641/boostfilesystempath-and-fopen
patch from 
https://github.com/DennisOSRM/Project-OSRM/pull/998