qpid-cpp: fix build

The compilation broke due to the flag `-Werror=int-in-bool-context`
which caused several compilation errors with GCC v7. Disabling this
warning manually with `-Wno-error` in `NIX_CFLAGS_COMPILE` should be
fine.

This package experienced several radical changes as the entire python
build in `$src/management/python` was broken since the given Python
interpreter missed several needed modules (including
`pythonPackages.qpid-python`). As the CMake build tried to invoke the
affected `setup.py` manually and patched the shebangs with `disutil` and
caused non-functional executables, I split the package up into two
parts, the actual `qpid-cpp` lib and the Python module that will be
composed using `buildEnv`.

Furthermore I added myself as maintainer for the package as the diff
became quite huge and we should have more folks available to maintain
this.

See https://hydra.nixos.org/build/71519082/log
See tickets #36453 and #31747