Fix build of Python bindings with GCC 8

[AdamWill]

GCC 8 appears to be rather stricter about various issues to do
with type conversions and casts than GCC 7 was. This affects
OpenColorIO's Python bindings quite heavily, producing a large
number of warnings (which are converted to errors by -Werror)
and outright errors.

The changes here are almost all one of three basic types:

1. Many functions that become Python methods with no arguments
   (using the METH_NOARGS flag) did not include the expected second
   parameter in their signatures at all. METH_NOARGS does not
   prevent this second parameter being passed at all, it only
   ensures that it will always be NULL. It's still not technically
   correct to leave it out of the function signature; as a comment
   from 'yak' on https://stackoverflow.com/questions/10256315
   points out, there are situations where this could cause a crash.
   I've added the second parameter (with no name, per convention)
   to every one of these cases.

2. In several cases, classes specified a custom destructor, with
   a cast to the destructor type, which only takes a single
   parameter. However, the signatures for these destructor functions
   included two parameters, assuming that they'd get an 'args'
   parameter (they do not). I've corrected all these cases.

3. In several cases, classes specified custom str or repr
   methods. However, in the PyTypeObject structures for these
   classes, these methods were not cast to the reprfunc type, as
   they ought to be. I've added these casts.

There are two warnings I just can't get rid of with my limited
C++ knowledge. The Config class (in PyConfig.cpp) defines a
couple of methods that take kwargs as well as args. This is
done by setting the METH_KEYWORDS flag, which ultimately seems
to result in a cast from type PyCFunctionWithKeywords to
PyCFunction happening somewhere behind the scenes. There's some
discussion of this at https://stackoverflow.com/questions/9496753

GCC 8 does not like this cast - it causes a 'cast-function-type'
warning. I've messed around a bit with reinterpret_cast and
stuff, but didn't really understand precisely what I was doing
and didn't manage to find anything that got rid of the warnings.
So I just added -Wno-error=cast-function-type to the compiler
flags instead, which turns these back into warnings and allows
the compilation to succeed.

I've tested at least that the compilation succeeds with only
those two warnings, and I can import the Python module and
instantiate a few classes and examine their docstrings and stuff
with no apparent errors or crashes.

Signed-off-by: Adam Williamson awilliam@redhat.com

[ignatenkobrain]

That's why people SHOULD NOT use -Werror for production builds⁰.

[AdamWill]

Sigh - CI errors are because this is an entirely new warning in GCC 8, so GCC 7 (which the CI uses) doesn't recognize -Wno-error=cast-function-type. Will have to find a way to conditionalize the disabling.

[ignatenkobrain]

@AdamWill just make it to not use -Werror when doing production builds, so you won't need -Wno-error

[AdamWill]

It's not my project, so not my decision.

[AdamWill]

Stole an idea from openembedded to use GCC pragmas...this should work better I hope.

[AdamWill]

Note: upstream https://gcc.gnu.org/ml/gcc-patches/2018-02/msg01315.html / https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84531 should stop the remaining two warnings being emitted. Not sure what GCC version they'll land in. Thanks @davidmalcolm for the reference.

[hodoulp]

Good Job.