adeed gcc only guard for 4_nbody_bhtree
Added compiler guards for 4_nbody_bhtree example.
If C++ compiler is not GNU C++, build for 4_nbody_bhtree aborts gracefully, without disrupting the rest of the build process.
Tested on Linux/OSX, with ICC as default compiler and g++-4.9 as GNU_CXX compiler used by 4_nbody_bhtree