Explain difference between cdef and cpdef

Add casting to const

--HG--
rename : tests/compile/ext_attribute_cache.pyx => tests/run/ext_attribute_cache.pyx

enable Py_TPFLAGS_HAVE_VERSION_TAG on extension types by default (also in Py2) and provide 'type_version_tag' option to disable it

Correctly handle function pointer typedefs with calling convention specified.

Fix extern variable cimport code in cimport_from_pyx mode

Consistently declare local error reporting variables (lineno, clineno, filename)

In cimport_from_pyx mode don't treat "cdef extern from" variables as being defined, preventing erroneous cross-module cimport.

Consistently declare local error reporting variables (lineno, clineno, filename) for all types of functions.

Fix C/C++ compilation error when if/else expression has extension class type.

The included test is derived from a real world case
which would lead to warnings in GCC if the pointer
wouldn't be explicitly casted to const.