warn when taking char* from a global Python variable

replace plain '#define CYTHON_PEP393_ENABLED' by flag #define as 1 or 0 to make it usable from Cython code

Fix compile error with gcc 4.7 and using c++11

For the new c++ standard a space between simple string concatenation
is mandatory, otherwise it's interpreted as operator"".

Fix a couple of C++ warnings.

Regression fixes

Conflicts:
	tests/run/numpy_common.pxi

more optimistic PyList_Append() implementation

restrict __Pyx_PyList_Append() usage to listcomps as it does not handle the case of shrinking the list array