Add back the old imagemagick under the name imagemagick@6

Formula/imagemagick@6.rb

@@ -0,0 +1,133 @@
+class ImagemagickAT6 < Formula
+  desc "Tools and libraries to manipulate images in many formats"
+  homepage "https://www.imagemagick.org/"
+  # Please always keep the Homebrew mirror as the primary URL as the
+  # ImageMagick site removes tarballs regularly which means we get issues
+  # unnecessarily and older versions of the formula are broken.
+  url "https://dl.bintray.com/homebrew/mirror/imagemagick-6.9.7-4.tar.xz"
+  mirror "https://www.imagemagick.org/download/ImageMagick-6.9.7-4.tar.xz"
+  sha256 "68842c55ed9c958b84aae17974961cefff4212bf7146f09fd15c09dbdc2d9629"
+
+  keg_only "Older version of imagemagick"
+
+  option "with-fftw", "Compile with FFTW support"
+  option "with-hdri", "Compile with HDRI support"
+  option "with-opencl", "Compile with OpenCL support"
+  option "with-openmp", "Compile with OpenMP support"
+  option "with-perl", "Compile with PerlMagick"
+  option "with-quantum-depth-8", "Compile with a quantum depth of 8 bit"
+  option "with-quantum-depth-16", "Compile with a quantum depth of 16 bit"
+  option "with-quantum-depth-32", "Compile with a quantum depth of 32 bit"
+  option "without-magick-plus-plus", "disable build/install of Magick++"
+  option "without-modules", "Disable support for dynamically loadable modules"
+  option "without-threads", "Disable threads support"
+  option "with-zero-configuration", "Disables depending on XML configuration files"
+
+  deprecated_option "enable-hdri" => "with-hdri"
+  deprecated_option "with-jp2" => "with-openjpeg"
+
+  depends_on "pkg-config" => :build
+  depends_on "libtool" => :run
+  depends_on "xz"
+
+  depends_on "jpeg" => :recommended
+  depends_on "libpng" => :recommended
+  depends_on "libtiff" => :recommended
+  depends_on "freetype" => :recommended
+
+  depends_on "fontconfig" => :optional
+  depends_on "little-cms" => :optional
+  depends_on "little-cms2" => :optional
+  depends_on "libwmf" => :optional
+  depends_on "librsvg" => :optional
+  depends_on "liblqr" => :optional
+  depends_on "openexr" => :optional
+  depends_on "ghostscript" => :optional
+  depends_on "webp" => :optional
+  depends_on "openjpeg" => :optional
+  depends_on "fftw" => :optional
+  depends_on "pango" => :optional
+  depends_on :perl => ["5.5", :optional]
+
+  needs :openmp if build.with? "openmp"
+
+  skip_clean :la
+
+  def install
+    args = %W[
+      --disable-osx-universal-binary
+      --prefix=#{prefix}
+      --disable-dependency-tracking
+      --disable-silent-rules
+      --enable-shared
+      --enable-static
+    ]
+
+    if build.without? "modules"
+      args << "--without-modules"
+    else
+      args << "--with-modules"
+    end
+
+    if build.with? "opencl"
+      args << "--enable-opencl"
+    else
+      args << "--disable-opencl"
+    end
+
+    if build.with? "openmp"
+      args << "--enable-openmp"
+    else
+      args << "--disable-openmp"
+    end
+
+    if build.with? "webp"
+      args << "--with-webp=yes"
+    else
+      args << "--without-webp"
+    end
+
+    if build.with? "openjpeg"
+      args << "--with-openjp2"
+    else
+      args << "--without-openjp2"
+    end
+
+    args << "--without-gslib" if build.without? "ghostscript"
+    args << "--with-perl" << "--with-perl-options='PREFIX=#{prefix}'" if build.with? "perl"
+    args << "--with-gs-font-dir=#{HOMEBREW_PREFIX}/share/ghostscript/fonts" if build.without? "ghostscript"
+    args << "--without-magick-plus-plus" if build.without? "magick-plus-plus"
+    args << "--enable-hdri=yes" if build.with? "hdri"
+    args << "--without-fftw" if build.without? "fftw"
+    args << "--without-pango" if build.without? "pango"
+    args << "--without-threads" if build.without? "threads"
+    args << "--with-rsvg" if build.with? "librsvg"
+    args << "--without-x" if build.without? "x11"
+    args << "--with-fontconfig=yes" if build.with? "fontconfig"
+    args << "--with-freetype=yes" if build.with? "freetype"
+    args << "--enable-zero-configuration" if build.with? "zero-configuration"
+
+    if build.with? "quantum-depth-32"
+      quantum_depth = 32
+    elsif build.with?("quantum-depth-16") || build.with?("perl")
+      quantum_depth = 16
+    elsif build.with? "quantum-depth-8"
+      quantum_depth = 8
+    end
+    args << "--with-quantum-depth=#{quantum_depth}" if quantum_depth
+
+    # versioned stuff in main tree is pointless for us
+    inreplace "configure", "${PACKAGE_NAME}-${PACKAGE_VERSION}", "${PACKAGE_NAME}"
+    system "./configure", *args
+    system "make", "install"
+  end
+
+  test do
+    assert_match "PNG", shell_output("#{bin}/identify #{test_fixtures("test.png")}")
+    # Check support for recommended features and delegates.
+    features = shell_output("#{bin}/convert -version")
+    %w[Modules freetype jpeg png tiff].each do |feature|
+      assert_match feature, features
+    end
+  end
+end