doctor: add xcode 7 SDK check

Library/Homebrew/cmd/doctor.rb

@@ -573,6 +573,19 @@ class Checks
     end
   end
 
+  # Xcode 7 lacking the 10.10 SDK is forcing sysroot to be declared
+  # nil on 10.10 & breaking compiles. CLT is workaround.
+  def check_sdk_path_not_nil_yosemite
+    if MacOS.version == :yosemite && MacOS::Xcode.installed? && MacOS.sdk_path.nil?
+      <<-EOS.undent
+      Xcode 7 lacks the 10.10 SDK which can cause some builds to fail.
+      We recommend installing the Command Line Tools with:
+        xcode-select --install
+      to resolve this issue.
+     EOS
+    end
+  end
+
   def check_user_path_1
     $seen_prefix_bin = false
     $seen_prefix_sbin = false