Making the core library compile with bgclang
Fixed #931: Make HPX work on BG/Q

This commit enables HPX on the BG/Q. It contains the proper selection
of user level context switch, proper timing methods and automatic
disabling of the TCPIP parcelport (non functional on BG/Q) as well as
automatic enabling of the MPI parcelport.

To compile HPX on the BG/Q you need bgclang and set the CMake variable
HPX_PLATFORM to BlueGeneQ.