gnu: imagemagick/fixed: Redirect old sonames to new sonames.

* gnu/packages/imagemagick.scm (imagemagick/fixed)[arguments]: Add
'fix-compat-cheat-rename-so phase to redirect old soname paths (expected
without grafting) to new sonames introduced by ImageMagick 6.9.12-0 and
later. These sonames are probably not forward compatible but most probably
backwards compatible so it should suffice until we remove the graft.

gnu/packages/imagemagick.scm

@@ -143,7 +143,29 @@ text, lines, polygons, ellipses and Bézier curves.")
                                   "6.9.12-2.tar.xz"))
               (sha256
                (base32
-                "17da5zihz58qm41y61sbvw626m5xfwr2nzszlikrvxyq1j1q7asa"))))))
+                "17da5zihz58qm41y61sbvw626m5xfwr2nzszlikrvxyq1j1q7asa"))))
+    (arguments
+     (substitute-keyword-arguments (package-arguments imagemagick)
+       ((#:phases phases)
+        `(modify-phases ,phases
+           (add-after 'install 'fix-compat-cheat-rename-so
+             (lambda* (#:key outputs #:allow-other-keys)
+               (with-directory-excursion
+                   (string-append (assoc-ref outputs "out")
+                                  "/lib")
+                 (symlink "libMagick++-6.Q16.so.9.0.0"
+                          "libMagick++-6.Q16.so.8.0.0")
+                 (symlink "libMagick++-6.Q16.so.9"
+                          "libMagick++-6.Q16.so.8")
+                 (symlink "libMagickCore-6.Q16.so.7.0.0"
+                          "libMagickCore-6.Q16.so.6.0.0")
+                 (symlink "libMagickCore-6.Q16.so.7"
+                          "libMagickCore-6.Q16.so.6")
+                 (symlink "libMagickWand-6.Q16.so.7.0.0"
+                          "libMagickWand-6.Q16.so.6.0.0")
+                 (symlink "libMagickWand-6.Q16.so.7"
+                          "libMagickWand-6.Q16.so.6"))
+               #t))))))))
 
 (define-public perl-image-magick
   (package