Expose Subversion revision number (calculated via "svnversion .") to Python.

Expose Subversion revision number (calculated via "svnversion .") to Python.
Add C API function Py_GetBuildNumber(), add it to the interactive prompt
banner (i.e. Py_GetBuildInfo()), and add it as the sys.build_number
attribute.  The build number is a string instead of an int because it may
contain a trailing 'M' if there are local modifications.