Doctor check for unlicensed Xcode

Checking the license text is probably the most future proofed method. Though for future reference other possible methods are listed in the below ticket.

Closes Homebrew/homebrew#14558.

Library/Homebrew/cmd/doctor.rb

@@ -925,6 +925,16 @@ def check_os_version
   end
 end
 
+  def check_xcode_license_approved
+    # If the user installs Xcode-only, they have to approve the
+    # license or no "xc*" tool will work.
+    <<-EOS.undent if `/usr/bin/xcrun clang 2>&1` =~ /license/ and not $?.success?
+    You have not agreed to the Xcode license.
+    Builds will fail! Agree to the license by opening Xcode.app or running:
+        xcodebuild -license
+    EOS
+  end
+
 end # end class Checks
 
 module Homebrew extend self

Library/Homebrew/cmd/install.rb

@@ -37,8 +37,11 @@ module Homebrew extend self
 
   def check_xcode
     require 'cmd/doctor'
-    xcode = Checks.new.check_for_latest_xcode
+    checks = Checks.new
-    opoo xcode unless xcode.nil?
+    %w{check_for_latest_xcode check_xcode_license_approved}.each do |check|
+      out = checks.send(check)
+      opoo out unless out.nil?
+    end
   end
 
   def check_macports