pythonmagick: fix build (#46469)

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.

lib/licenses.nix

@@ -355,6 +355,11 @@ lib.mapAttrs (n: v: v // { shortName = n; }) rec {
     fullName = "Independent JPEG Group License";
   };
 
+  imagemagick = spdx {
+    fullName = "ImageMagick License";
+    spdxId = "imagemagick";
+  };
+
   inria-compcert = {
     fullName  = "INRIA Non-Commercial License Agreement for the CompCert verified compiler";
     url       = "http://compcert.inria.fr/doc/LICENSE";