Ensure that PY_ARRAY_UNIQUE_SYMBOL is uniquely defined for each extension.

By using a unique symbol for each extension we prevent multiple
definition linking errors which arise when statically linking
a Python interpreter.

Closes gh-2039.
Merge pull request #2042 from bfroehle/py_array_unique_symbol

Ensure that PY_ARRAY_UNIQUE_SYMBOL is uniquely defined for each extension