Enable compilation with older g++

[Hopobcn]

In CUDA 7.5 nvcc only supports up to g++ 4.9 therefore in systems where g++ expands to g++-5 -ccbin=g++-4.9 has to be provided to nvcc.

If not the compiler fails:

$ make
Grabbing  src/nccl.h                > build/include/nccl.h     
Compiling src/libwrap.cu            > build/obj/libwrap.o      
Compiling src/core.cu               > build/obj/core.o         
Compiling src/all_gather.cu         > build/obj/all_gather.o   
/usr/lib/gcc/x86_64-linux-gnu/5/include/mwaitxintrin.h(36): error: identifier "__builtin_ia32_monitorx" is undefined

/usr/lib/gcc/x86_64-linux-gnu/5/include/mwaitxintrin.h(42): error: identifier "__builtin_ia32_mwaitx" is undefined

2 errors detected in the compilation of "/tmp/tmpxft_00003a6e_00000000-13_all_gather.compute_52.cpp1.ii".
Makefile:101: recipe for target 'build/obj/all_gather.o' failed
make: *** [build/obj/all_gather.o] Error 2

With my changes you may compile using:
GPP=g++-4.9 make CUDA_HOME=<cuda install path> test

[root29]

I use 'GPP=g++-4.9 make CUDA_HOME=</usr/local/cuda> test'
but still error. Do you know the reason? Thanks!

[sjeaugey]

GPP is no longer defined in the makefile, it has been replaced by the standard CXX.