Conditionnal OpenMP validation

Disable OpenMP validation if OpenMP is not supported by the underlying
compiler (e.g. clang++)
Do not try to infer type from assigned slice/ext slice
Refactor pythran namespace naming

Funny bug: previous value once led to the namespace __pascal that was
used by clang...
Simplify Type inference

To the price of a small regression (see new skipped test), compilation
times are now reasonable with gcc.

The key is to *not* perform two typing passes but only one.
Full clang compatibility

After a few changes in the core::list code, pythonic is now compatible
with clang.

Most issues were related to implicit casts leading to ambiguous
conversions...

pythran/tests/cases/quicksort.py led to the return of uninitialized
value,but their is already an issue opened for this case.