Fix Xcode builds on 10.9.

CLT-only builds are still broken and need to be fixed at some point.

Closes Homebrew/homebrew#20524.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/os/mac/xcode.rb

@@ -158,8 +158,12 @@ module MacOS::CLT extend self
   # This is true ift he standard UNIX tools are present under /usr. For
   # Xcode < 4.3, this is the standard location. Otherwise, it means that
   # the user has installed the "Command Line Tools for Xcode" package.
+  # TODO: handle 10.9 CLT which has headers under:
+  # /Library/Developer/CommandLineTools/usr/include
   def installed?
-    MacOS.dev_tools_path == Pathname.new("/usr/bin")
+    return false if MacOS.version > :mountain_lion
+    MacOS.dev_tools_path == Pathname.new("/usr/bin") and
+      File.directory? "/usr/include"
   end
 
   def latest_version?

Library/Homebrew/superenv/macsystem.rb

 # new code because I don't really trust the Xcode code now having researched it more
 module MacSystem extend self
   def xcode_clt_installed?
-    File.executable? "/usr/bin/clang" and File.executable? "/usr/bin/lldb" and File.executable? "/usr/bin/make"
+    # TODO: handle 10.9 CLT which has headers under:
+    # /Library/Developer/CommandLineTools/usr/include
+    File.executable? "/usr/bin/clang" and File.executable? "/usr/bin/lldb" and
+      File.executable? "/usr/bin/make" and File.directory? "/usr/include"
   end
 
   def xcode43_without_clt?