Document (and possibly optimize) compilation flags

[odelalleau]

Çağlar Gülçehre reported on the mailing list compilation flags that can boost performance on the CPU:
http://groups.google.com/group/theano-users/browse_thread/thread/0f7045ae8233ae37/17c6828139298050
It would be good to have such information in the documentation.

Even better: add a way to automatically optimize compilation flags to minimize execution time of a given Theano function.

[jaberg]

I don't think anyone ever made an effort to optimize the choice of default flags. There might be no incentive to do function-specific optimization. Lets look at this next time we do some benchmarking.

[nouiz]

I added documentation in the tutorial faq. that give a more recent version of those flags.

[nouiz]

native is done by default in a good way. The other one are documented.