gf: imfreq. general case, fix python interface

- Fix constructor, now taking n_points like the positive_only case
- Fix python, h5read, for general case (i.e. all frequencies).
shared_ptr<void> can not be constructed directly in c++14

- Fix for compilation on clang 3.4 in c++1y mode.
Put back the check of compiler version commented for unknown reasons...

Probably a mistake ....
continued : further correction on mac

The issue was the detection of clang version on mac,
depends if packaged by Apple or not...