package/homebrew: point MAGNUM_PLUGINS_DIR to proper location

Homebrew sets CMAKE_INSTALL_PREFIX to e.g.
/usr/local/Cellar/magnum/HEAD-920db7a/lib/magnum/importers and not the
final symlinked location, which then confuses the plugin location
autodetection. This should fix it.

package/homebrew/magnum.rb

@@ -13,7 +13,7 @@ class Magnum < Formula
   def install
     system "mkdir build"
     cd "build" do
-      system "cmake", "-DCMAKE_BUILD_TYPE=Release", "-DCMAKE_INSTALL_PREFIX=#{prefix}", "-DWITH_AUDIO=ON", "-DWITH_GLFWAPPLICATION=OFF", "-DWITH_SDL2APPLICATION=ON", "-DWITH_WINDOWLESSCGLAPPLICATION=ON", "-DWITH_CGLCONTEXT=ON", "-DWITH_OPENGLTESTER=ON", "-DWITH_ANYAUDIOIMPORTER=ON", "-DWITH_ANYIMAGECONVERTER=ON", "-DWITH_ANYIMAGEIMPORTER=ON", "-DWITH_ANYSCENEIMPORTER=ON", "-DWITH_MAGNUMFONT=ON", "-DWITH_MAGNUMFONTCONVERTER=ON", "-DWITH_OBJIMPORTER=ON", "-DWITH_TGAIMAGECONVERTER=ON", "-DWITH_TGAIMPORTER=ON", "-DWITH_WAVAUDIOIMPORTER=ON", "-DWITH_DISTANCEFIELDCONVERTER=ON", "-DWITH_FONTCONVERTER=ON", "-DWITH_IMAGECONVERTER=ON", "-DWITH_MAGNUMINFO=ON", "-DWITH_AL_INFO=ON", ".."
+      system "cmake", "-DCMAKE_BUILD_TYPE=Release", "-DCMAKE_INSTALL_PREFIX=#{prefix}", "-DMAGNUM_PLUGINS_DIR=#{HOMEBREW_PREFIX}/lib/magnum", "-DWITH_AUDIO=ON", "-DWITH_GLFWAPPLICATION=OFF", "-DWITH_SDL2APPLICATION=ON", "-DWITH_WINDOWLESSCGLAPPLICATION=ON", "-DWITH_CGLCONTEXT=ON", "-DWITH_OPENGLTESTER=ON", "-DWITH_ANYAUDIOIMPORTER=ON", "-DWITH_ANYIMAGECONVERTER=ON", "-DWITH_ANYIMAGEIMPORTER=ON", "-DWITH_ANYSCENEIMPORTER=ON", "-DWITH_MAGNUMFONT=ON", "-DWITH_MAGNUMFONTCONVERTER=ON", "-DWITH_OBJIMPORTER=ON", "-DWITH_TGAIMAGECONVERTER=ON", "-DWITH_TGAIMPORTER=ON", "-DWITH_WAVAUDIOIMPORTER=ON", "-DWITH_DISTANCEFIELDCONVERTER=ON", "-DWITH_FONTCONVERTER=ON", "-DWITH_IMAGECONVERTER=ON", "-DWITH_MAGNUMINFO=ON", "-DWITH_AL_INFO=ON", ".."
       system "cmake", "--build", "."
       system "cmake", "--build", ".", "--target", "install"
     end