libid3tag: create pkg-config file

Resolves following issue when compiling external applications against
libid3tag.

    Package id3tag was not found in the pkg-config search path.
    Perhaps you should add the directory containing `id3tag.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'id3tag' found

Closes Homebrew/homebrew#7973.

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

Formula/libid3tag.rb

@@ -4,6 +4,24 @@ class Libid3tag < Formula
   url 'http://downloads.sourceforge.net/project/mad/libid3tag/0.15.1b/libid3tag-0.15.1b.tar.gz'
   md5 'e5808ad997ba32c498803822078748c3'
 
+  def id3tag_pc
+    return <<-EOS
+prefix=#{HOMEBREW_PREFIX}
+exec_prefix=${prefix}
+libdir=${exec_prefix}/lib
+includedir=${prefix}/include
+
+Name: id3tag
+Description: ID3 tag reading library
+Version: #{@version}
+Requires:
+Conflicts:
+Libs: -L${libdir} -lid3tag -lz
+Cflags: -I${includedir}
+    EOS
+  end
+
+
   def homepage
     Formula.factory('mad').homepage
   end
@@ -11,5 +29,7 @@ class Libid3tag < Formula
   def install
     system "./configure", "--prefix=#{prefix}", "--disable-debug", "--disable-dependency-tracking"
     system "make install"
+
+    (lib+'pkgconfig/id3tag.pc').write id3tag_pc
   end
 end