Never trigger too old CLT check on <10.7.

While we fake a CLT for these versions they never shipped with the CLT
so cannot update it.

Library/Homebrew/extend/os/mac/diagnostic.rb

@@ -116,7 +116,7 @@ module Homebrew
 
       def check_xcode_minimum_version
         return unless MacOS::Xcode.installed?
-        return unless MacOS::Xcode.minimum_version?
+        return unless MacOS::Xcode.below_minimum_version?
 
         <<-EOS.undent
           Your Xcode (#{MacOS::Xcode.version}) is too outdated.
@@ -127,7 +127,7 @@ module Homebrew
 
       def check_clt_minimum_version
         return unless MacOS::CLT.installed?
-        return unless MacOS::CLT.minimum_version?
+        return unless MacOS::CLT.below_minimum_version?
 
         <<-EOS.undent
           Your Command Line Tools are too outdated.

Library/Homebrew/os/mac/xcode.rb

@@ -32,7 +32,7 @@ module OS
         end
       end
 
-      def minimum_version?
+      def below_minimum_version?
         version < minimum_version
       end
 
@@ -231,11 +231,16 @@ module OS
         end
       end
 
-      def minimum_version?
+      def below_minimum_version?
+        # Lion was the first version of OS X to ship with a CLT
+        return false if MacOS.version < :lion
         version < minimum_version
       end
 
       def outdated?
+        # Lion was the first version of OS X to ship with a CLT
+        return false if MacOS.version < :lion
+
         if MacOS.version >= :mavericks
           version = Utils.popen_read("#{MAVERICKS_PKG_PATH}/usr/bin/clang --version")
         else