Remove duplicate computation and compilation.

...rst time

As a side effect don't always loop over user provided cxx flags always when it contain march.

This lower the number of compilation and help to don't get the compile lock for following compilation of the same graph!

NEWS.txt:
- use the march flags for all compilation in a process, not just for the first one. (Frederic B.)
  This compile less code and should ask for the compile lock less frequently
- Don't duplicate elemwise computation due to broadcasting. (Frederic B., reported by Marcel Stimberg)