Rely on USE_BOOST_SIMD instead of __AVX__ to activate boost.simd

As a consequence, it is now fully legal to use -march=native as a
compiler flag.
Update default compiler options

-fno-math-errno gives some speedups and errno is not used in pythran
-march=native does not yield wonderful speedups, but may enable extra
vectorization
Harmonize True, False and None handling

They should be part of __builtin__ instead of receiving a special
treatment.

None is also const, which implies a few minor changes.
Make all constructors regular member of __builtin__

This avoids extra processing and harmonize the whole function handling.
Merge pull request #144 from serge-sans-paille/builtin-constructor

Builtin constructor