ImageMagick on Mac requesting incompatible library version

I have a test site installed on my Mac workstation. When I try to configure ImageMagick as toolkit I get the following error:
User error: ImageMagick error 5: dyld: Library not loaded: /opt/local/lib/libltdl.7.dylib Referenced from: /opt/local/bin/convert Reason: Incompatible library version: convert requires version 11.0.0 or later, but libltdl.7.dylib provides version 9.0.0 in _imagemagick_convert_exec() (line 519 of /Users/joelguesclin/Sites/acquia-drupal/sites/all/modules/imagemagick/imagemagick.module).
I have just upgraded to Mavericks (it worked fine before that) and have upgraded ImageMagick to latest version, here is details:

imac:~ joelguesclin$ /opt/local/bin/convert --version
Version: ImageMagick 6.8.8-3 Q16 x86_64 2014-02-09 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2014 ImageMagick Studio LLC
Features: DPC Modules
Delegates: bzlib djvu fftw fontconfig freetype gslib jng jpeg lcms ltdl lzma png ps tiff webp x xml zlib

When I run this:

imac:~ joelguesclin$ /opt/local/bin/convert logo.gif logo.jpeg
imac:~ joelguesclin$ display logo.jpeg

everything works fine, no problem.
I do not understand Mac configuration in its finer detail, but it seems to me that the problem must lie somewhere in Drupal rather than in ImageMagick, otherwise I would expect the convert command to fail