To support building Csound 5 on Cygwin, and to make it easier to support
building on Windows with Microsoft .NET SDK, I have made the following
changes:

1. Added custom.py, which is parsed by SConstruct to produce customized
options such as include paths or lib paths on platforms like Windows and
mingw where these are not standardized.

2. Changed Top/opcode.c to #include "H/opcode.h" not "opcode.h", to prevent
a conflict with Python's own opcode.h when building with custom.py.

3. Added cygwin_import_libs/libpython23.a, which is a Cygwin import library
for the Python shared library that comes with the distribution from
www.python.org and that is built using Microsoft Visual C++. custom.py sets
a LIBPATH for this on Cygwin and mingw.