Fix variable name in doc.

Fix a mismatched variable name in the "Sharing Extention Types"
documentation.

Atomic acquisition counting, slice assignment (+scalars), overlapping memory, broadcasting, more memoryview object methods, optimized with gil block

Fix C++ reference to C++ reference coercion

with some improvements to previously committed test case

This commit allows you to pass in variables for the conditional compilation
feature from distutils. This allows you to do fairly complex autoconf-type
checks from setup.py and pass the results in to the compiler.

This is an update to the change in ticket #323 here:
http://trac.cython.org/cython_trac/ticket/323