yaz: Another way to close Homebrew#167.

Library/Formula/yaz.rb

@@ -8,6 +8,17 @@ class Yaz < Formula
   depends_on 'pkg-config' => :build
 
   def install
+    # Both libxml2 2.7.3 and icu are defined the same data type `UChar'.
+    ENV.append_to_cflags "-Wno-typedef-redefinition" if ENV.compiler == :clang
+
+    if (ENV.compiler == :gcc or ENV.compiler == :llvm) and build.include? "env=std"
+      opoo <<-EOS.undent
+        GCC and LLVM-GCC can't build with icu and system's libxml together.
+        If you have request about this and have Clang, you can compile it with:
+          brew install yaz --env=std --use-clang
+      EOS
+    end
+
     system "./configure", "--disable-dependency-tracking",
                           "--prefix=#{prefix}",
                           "--with-xml2"