The original build broke with the following linker issue:
```
CXXLD _PythonMagick.la
/nix/store/h0lbngpv6ln56hjj59i6l77vxq25flbz-binutils-2.30/bin/ld: cannot find -l-L/nix/store/4gh6ynzsd5ndx37hmkl62xa8z30k43y1-imagemagick-6.9.9-34/lib
collect2: error: ld returned 1 exit status
```
This happens since `BOOST_PYTHON_LIB` wasn't set properly, however
`_PythonMagick.la` was linked with `-l$(BOOST_PYTHON_LIB)
$(MAGICK_LIBS)`. With an empty `BOOST_PYTHON_LIB` the linker got
confused.
To work around this, the `boost` library directory needs to be specified
explicitly. To ensure that the changes take effect, the original
`configure` script shipped with `$src` needs to be removed and recreated
using the `autoreconfHook`.
Additionally the `imagemagick` license (https://spdx.org/licenses/ImageMagick.html)
needs to be added to `lib/licenses.nix` to document the proper license
of `pythonmagick` in the meta section.
