sip: fix for Xcode-only install

- Pass in CFLAGS and LDFLAGS. Note, LDFLAGS have to be passed into
  configure.py as "LFLAGS".

Closes Homebrew#12884.

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

Library/Formula/sip.rb

@@ -34,7 +34,9 @@ def install
     system "python", "configure.py",
                               "--destdir=#{lib}/#{which_python}/site-packages",
                               "--bindir=#{bin}",
-                              "--incdir=#{include}"
+                              "--incdir=#{include}",
+                              "CFLAGS=#{ENV.cflags}",
+                              "LFLAGS=#{ENV.ldflags}"
     system "make install"
   end