Imagemagick: patch for xml2-config

Imagemagick-6.7.x uses libxml2, but Apple changed the file
`xml2-config` starting in Mt. Lion, removing the `--prefix` flag.
That breaks Imagemagick, which uses it to find the library.
Because all versions of OSX have libxml2 in the same location,
this commit is safe for all users.

* Patch `configure` to not run `xml2-config --prefix` by placing
the value it returns, `/usr`, into the corresonding variable.

Fixes #11789.
Closes #11883.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Formula/imagemagick.rb

@@ -84,6 +84,13 @@ def skip_clean? path
     path.extname == '.la'
   end
 
+  def patches
+    # Fixes xml2-config that can be missing --prefix.  See issue #11789
+    # Remove if the final Mt. Lion xml2-config supports --prefix.
+    # Not reporting this upstream until the final Mt. Lion is released.
+    DATA
+  end
+
   def options
     [
       ['--with-ghostscript', 'Compile against ghostscript (not recommended.)'],
@@ -148,3 +155,16 @@ def test
     system "#{bin}/identify", "/Library/Application Support/Apple/iChat Icons/Flags/Argentina.gif"
   end
 end
+
+__END__
+--- a/configure	2012-02-25 09:03:23.000000000 -0800
++++ b/configure	2012-04-26 03:32:15.000000000 -0700
+@@ -31924,7 +31924,7 @@
+         # Debian installs libxml headers under /usr/include/libxml2/libxml with
+         # the shared library installed under /usr/lib, whereas the package
+         # installs itself under $prefix/libxml and $prefix/lib.
+-        xml2_prefix=`xml2-config --prefix`
++        xml2_prefix=/usr
+         if test -d "${xml2_prefix}/include/libxml2"; then
+             CPPFLAGS="$CPPFLAGS -I${xml2_prefix}/include/libxml2"
+         fi