title: Octave can't read file using GraphicsMagick
body: <p>I'm trying to load an image in Octave 4.2.1 with GraphicsMagick 1.3.27 (via brew) on OS X. Here's the error:</p>
  
  <pre><code>&gt;&gt; img = imread('./myimg.jpg')
  error: Magick++ exception: octave-cli-4.2: No decode delegate for this 
  image format (&lt;path to myimg.jpg&gt;) reported by magick/constitute.c:1535 (ReadImage)
   error: called from
      __imread__ at line 80 column 10
      imageIO at line 117 column 26
      imread at line 106 column 30
  </code></pre>
  
  <p>GraphicsMagick looks to be configured correctly:</p>
  
  <pre><code> gm -version
   GraphicsMagick 1.3.27  Q16 http://www.GraphicsMagick.org/
   Copyright (C) 2002-2017 GraphicsMagick Group.
   Additional copyrights and licenses apply to this software.
   See http://www.GraphicsMagick.org/www/Copyright.html for details.
  
   Feature Support:
     Native Thread Safe       yes
     Large Files (&gt; 32 bit)   yes
     Large Memory (&gt; 32 bit)  yes
     BZIP                     yes
     DPS                      no
     FlashPix                 no
     FreeType                 yes
     Ghostscript (Library)    no
     JBIG                     no
     JPEG-2000                no
     JPEG                     yes
     Little CMS               no
     Loadable Modules         yes
     OpenMP                   no
     PNG                      yes
     TIFF                     yes
     TRIO                     no
     UMEM                     no
     WebP                     no
     WMF                      no
     X11                      no
     XML                      no
     ZLIB                     yes
  
   Host type: x86_64-apple-darwin16.7.0
  
   Configured using the command:
     ./configure  '--prefix=/usr/local/Cellar/graphicsmagick/1.3.27' '-- 
  disable-dependency-tracking' '--enable-shared' '--disable-static' '--with-modules' '--without-lzma' '--disable-openmp' '--with-quantum-depth=16' '--disable-installed' '--without-gslib' '--with-gs-font-dir=/usr/local/share/ghostscript/fonts' '--without-x' '--without-lcms2' 'CC=clang' 'CXX=clang++'
  
   Final Build Parameters:
     CC       = clang
     CFLAGS   = -g -O2 -Wall -D_THREAD_SAFE
     CPPFLAGS = -I/usr/local/opt/freetype/include/freetype2
     CXX      = clang++
     CXXFLAGS = -D_THREAD_SAFE
     LDFLAGS  = -L/usr/local/opt/freetype/lib
     LIBS     = -lfreetype -lbz2 -lz -lltdl -lm -lpthread
  </code></pre>
  
  <p>Trying to load a png file results in the same error. Using <code>gm</code> directly from the command line works fine.</p>