libxml2: Fix Python bindings for Xcode-only

- Since libxml2 is a dupe, we have to assure to link against it and not system's python.
- Unless the CLT are present, apply a patch for the linking step.
- Help libxml2/python/setup.py to find libiconv headers.

Closes Homebrew/homebrew#13511.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/libxml2.rb

@@ -19,6 +19,15 @@ class Libxml2 < Formula
     ]
   end
 
+  def patches
+    # Libxml2 is a dupe and now we see why that's a bad idea.
+    # Python's distutils are so smart to remember the LD command from
+    # when python was built and therefore finds the libxml2 in Xcode first.
+    # The LDFLAGS don't help, because python puts them _after_ the remembered
+    # flags.
+    DATA unless MacOS::CLT.installed?
+  end
+
   def install
     ENV.universal_binary if ARGV.build_universal?
 
@@ -41,6 +50,11 @@ class Libxml2 < Formula
         ENV.append 'CFLAGS', arch_flags
         ENV.append 'LDFLAGS', arch_flags
 
+        unless MacOS::CLT.installed?
+          # For Xcode-only systems, the libiconv headers are inside of Xcode.
+          # We can replace /opt/include with our path to achieve that
+          inreplace 'setup.py', '"/opt/include",', "\"#{MacOS.sdk_path}/usr/include\","
+        end
         system "python", "setup.py", "install_lib",
           "--install-dir=#{python_lib}"
       end
@@ -51,3 +65,19 @@ class Libxml2 < Formula
     "python" + `python -c 'import sys;print(sys.version[:3])'`.strip
   end
 end
+
+__END__
+diff --git a/python/setup.py.in b/python/setup.py.in
+index b985979..d6ccf2e 100755
+--- a/python/setup.py.in
++++ b/python/setup.py.in
+@@ -13,6 +13,9 @@ ROOT = r'@prefix@'
+ # Thread-enabled libxml2
+ with_threads = @WITH_THREADS@
+
++import distutils.sysconfig as sc
++sc.get_config_vars()['LDSHARED']=os.environ['LD'] + ' -bundle -undefined dynamic_lookup -L' + ROOT + '/lib ' + os.environ['LDFLAGS']
++
+ # If this flag is set (windows only),
+ # a private copy of the dlls are included in the package.
+ # If this flag is not set, the libxml2 and libxslt