brew-doctor - check for another stray ENV var

Having $MACOSX_DEPLOYMENT_TARGET (for Fink) to something other than
the current system SDK will cause Git to fail to compile (when building
the Python bindings).

Library/Homebrew/brew_doctor.rb

@@ -568,6 +568,21 @@ def check_for_linked_kegonly_brews
   end
 end
 
+def check_for_other_vars
+  target_var = ENV['MACOSX_DEPLOYMENT_TARGET']
+  return if target_var.nil? or target_var.empty?
+
+  unless target_var == MACOS_VERSION.to_s
+    puts <<-EOS.undent
+    $MACOSX_DEPLOYMENT_TARGET was set to #{target_var}
+    This is used by Fink, but having it set to a value different from the
+    current system version (#{MACOS_VERSION}) can cause problems, compiling
+    Git for instance, and should probably be removed.
+
+    EOS
+  end
+end
+
 def brew_doctor
   read, write = IO.pipe
 
@@ -595,6 +610,7 @@ def brew_doctor
     check_for_gettext
     check_for_config_scripts
     check_for_dyld_vars
+    check_for_other_vars
     check_for_symlinked_cellar
     check_for_multiple_volumes
     check_for_git