Explicitly disable support for OpenEXR and DjVu by default. These were
never dependencies before, so official packages never contained the
support, but if a local system had them installed, ImageMagick would try
to use them -- and sometimes fail.
Also add a patch, that modifies the module-searching algorithm from
/always/ checking the compiled-in constant path, to only checking it, if
an environment variable is not set. This fixes a problem with testing,
where a module installed by an earlier version of the software could be
used by the newly built version, that's being tested.
This port has too many options -- perhaps, support for the most common
things (like PNG, JPEG, and GIF) should be permanent...