Fix macOS boost-python issues

[akashlevy]

What are the reasons/motivation for this change?

This PR resolves the issue below seen on my macOS Apple Silicon with a vanilla Yosys clone:

akashlevy@Akashs-MacBook-Pro yosys % make -j8 ENABLE_PYOSYS=1
$BREW_PREFIX is [/opt/homebrew/opt]
Makefile:343: *** BOOST_PYTHON_LIB could not be detected. Please define manually.  Stop.

There are two issues:

1. The boost include path is incorrect ($(BREW_PREFIX)/boost/include/boost instead of $(BREW_PREFIX)/boost/include). This means that the canonical form of Boost includes such as #include <boost/...> will not work
2. boost-python3 library needs to be linked to detect boost-python3 properly

You can work around these issues by manually setting the BOOST_PYTHON_LIB, but this PR makes it so that is not needed.

Explain how this is achieved.

1. Use $(BREW_PREFIX)/boost/include as the Boost include path
2. Add -L$(BREW_PREFIX)/boost-python3/lib to the LINKFLAGS and include LINKFLAGS in CHECK_BOOST_PYTHON

If applicable, please suggest to reviewers how they can test the change.

This was tested on an Apple Silicon macOS (M2) with Pyosys enabled and was able to properly compile. I do not have access to an Intel macOS, it would be great if this can be tested against some builds as well.

[KrystalDelusion]

Would you be willing to add a ci job for pyosys to the extra-builds.yml? If you use a strategy matrix you can specify both macos-latest (which uses M1) and macos-13 (which is Intel based); maybe throw in ubuntu-latest for well-roundedness.

[akashlevy]

Certainly, I can try to do that when I get time