The `-DBOOST_VERSION` flag has unnecessary and unwanted quotes

When providing the boost version as a compiler flag, we will obtain a compiler option -DBOOST_VERSION="a.b.c", following from here: https://gitlab.idiap.ch/bob/bob.extension/blob/b5c85f8b89bac3b4eee00490303fb8a7fc146b1a/bob/extension/boost.py#L213

While for most of the compilers this does not seem to be a problem, I had an issue when trying to compile and link bob libraries against cuda (nvcc) compiled libraries in Caffe, see: https://groups.google.com/forum/#!topic/bob-devel/TyWkD7pB0h4

I think, the quotes are completely unnecessary. We should be able to remove them and still compile Bob's libraries -- at least the above-mentioned compilation worked after removing the quotes.

Is there any possibility to test the compilation without the quotes?