Imagemagick-7.0.8 needs to link against libltdl. Otherwise, the build will fail with:
```
2 errors found in build log:
503 checking for libltdl...
504 checking ltdl.h usability... no
505 checking ltdl.h presence... no
506 checking for ltdl.h... no
507 checking for lt_dlinit in -lltdl... no
508 checking if libltdl package is complete... no
>> 509 configure: error: in `/tmp/gpjohnsn/spack-stage/spack-stage-imagemagick-7.0.8-7-4y44gaklhhciiwjzhfpxjfwdj5q
ltjp3/spack-src':
>> 510 configure: error: libltdl is required for modules and OpenCL builds
511 See `config.log' for more details
```
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