fix AS_HELP_STRING

  --enable-gtk-doc        use gtk-doc to build documentation [[default=no]]
  --enable-gtk-doc-html   build documentation in html format [[default=yes]]
  --enable-gtk-doc-pdf    build documentation in pdf format [[default=no]]
  --enable-image-convert  Enable image-converter plugin
Merge pull request #3 from NiceandGently/master

fix AS_HELP_STRING