Boost 1.67 Python components require a Python version suffix.
The logic for this was added to the CMake machinery of the upstream
library in casacore/casacore#846, but this won't make it into a release
for a while. Add a patch in the meantime, to be removed on the next
release.

The wcslib formula moved back to homebrew/core in July 2018 after
brewsci grew stale. The boost-python library for Python 3 has a
separate formula since 1.66.0 (Feb 2018). Restore gcc dependency
since casacore looks for a Fortran compiler during cmake configure.

Since casacore 3.0 it is now mandatory to use C++11 features (see
casacore/casacore#580). Also ensure that undefined symbols are
ignored until runtime in the Python shared library.

Only apply the patch to current releases (3.0 and 3.1). The HEAD and
future 3.2 release already has it (due to casacore/casacore#846).

These provide more robust detection of Python versions on macOS and
replace the functionality of casacore's FindPython.cmake and
FindNUMPY.cmake. Previously the Homebrew Python 3 include dirs were
confused with those of Homebrew Python 2. There is also no need to
hardcode the Python executable and library (always a bad idea).

While these modules are available since cmake 3.12, they could be
copied to casacore and still run on cmake 3.7, in case the bigger
project has need of them.

The stable patch turns v3.0.0 into c349f27, effectively applying #846.
The original patch only included the first commit of this PR (d'oh).
The cmake patch now applies on top of #846 and also works on the latest
HEAD.

The boost-python3 formula has no --with-python option (that sounds
tautological anyway).

Use latest stable version from GitHub.

Follow the "Python for Formula Authors" document, which states that
'Python 2 libraries do not need a depends_on "python@2" declaration;
they will be built with the system Python, but should still be usable
with any other Python 2.7.' Thereby make the optional python@2 support
mandatory, since system Python 2.7 is always available.

The HEAD no longer needs a patch (although it is currently running off
a feature branch to be merged as casacore/casacore#922). Now use a
single patch that turns v3.1.0 into HEAD (at least as far as the
Python build scripts are concerned).

Thanks to casacore/casacore#922 we can also drop all the patches.