Fix build on RHEL7 with GCC 4.8

Add -std=c++11 to CXXFLAGS.
Fixes #116.


[ROCm/rccl-tests commit: 365b92a1ea]

projects/rccl-tests/src/Makefile

@@ -35,6 +35,7 @@ NVCC_GENCODE ?= -gencode=arch=compute_35,code=sm_35 \
 endif
 
 NVCUFLAGS  := -ccbin $(CXX) $(NVCC_GENCODE) -std=c++11
+CXXFLAGS   := -std=c++11
 
 LDFLAGS    := -L${CUDA_LIB} -lcudart -lrt
 NVLDFLAGS  := -L${CUDA_LIB} -l${CUDARTLIB} -lrt