graphics/caffe: unbreak with boost 1.67

/usr/local/bin/ld: cannot find -lboost_python
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
gmake[1]: *** [Makefile:508: python/caffe/_caffe.so] Error 1

PR: 227427
Reported by: antoine (via exp-run)