Add keg-only reason symbols for Xcode 4.3 and Xcode 5

Closes Homebrew/homebrew#28095.

Formula/autoconf.rb

@@ -13,9 +13,7 @@ class Autoconf < Formula
     sha1 "7d31f63e5ddd1bbbf0397b0b70df1ff9e70f998b" => :lion
   end
 
-  if MacOS::Xcode.provides_autotools? or File.file? "/usr/bin/autoconf"
-    keg_only "Xcode (up to and including 4.2) provides (a rather old) Autoconf."
-  end
+  keg_only :provided_until_xcode43
 
   def install
     ENV['PERL'] = '/usr/bin/perl'

Formula/automake.rb

@@ -15,9 +15,7 @@ class Automake < Formula
   # Always needs a newer autoconf, even on Snow Leopard.
   depends_on 'autoconf' => :run
 
-  if MacOS::Xcode.provides_autotools? or File.file? "/usr/bin/automake"
-    keg_only "Xcode (up to and including 4.2) provides (a rather old) Automake."
-  end
+  keg_only :provided_until_xcode43
 
   def install
     ENV['PERL'] = '/usr/bin/perl'

Formula/bsdmake.rb

@@ -5,9 +5,7 @@ class Bsdmake < Formula
   url 'http://opensource.apple.com/tarballs/bsdmake/bsdmake-24.tar.gz'
   sha1 '9ce3c3fc01e0eb47d82827b1eb227eb371fefd5c'
 
-  if MacOS::Xcode.provides_autotools? or File.file? "/usr/bin/bsdmake"
-    keg_only "Recent versions of OS X no longer provide this tool."
-  end
+  keg_only :provided_until_xcode43
 
   # MacPorts patches to make bsdmake play nice with our prefix system
   # Also a MacPorts patch to circumvent setrlimit error

Formula/cvs.rb

@@ -10,7 +10,7 @@ class Cvs < Formula
   url 'http://ftp.gnu.org/non-gnu/cvs/source/feature/1.12.13/cvs-1.12.13.tar.bz2'
   sha1 '93a8dacc6ff0e723a130835713235863f1f5ada9'
 
-  keg_only "Xcode (< 5.0) provides CVS" if MacOS::Xcode.provides_cvs?
+  keg_only :provided_until_xcode5
 
   {
     "PR5178707"         => "372385b34a346753249a7808e8d5db0a6cadd3ee",

Formula/libtool.rb

@@ -16,9 +16,7 @@ class Libtool < Formula
     sha1 '6873a7b72e86f369f43125c0e29ae5cdbc2d67c1' => :lion
   end
 
-  if MacOS::Xcode.provides_autotools? or File.file? "/usr/bin/glibtoolize"
-    keg_only "Xcode 4.2 and below provide glibtool."
-  end
+  keg_only :provided_until_xcode43
 
   option :universal