Add guards to calls that would trigger Xcode install requests

add guard in Formula#file_modified? to prevent git popup

add guard in Superenv.bin before calling MacOS::Xcode.version

add guard against missing Xcode/CLT in Xcode.uncached_version

return nil instread of 0 in uncached_version when Xcode/CLT are not present, to distinguish from linuxbrew behavior

checks against pour_bottle? and needs_relocation?, add guard around keg.relocate_install_names to check pour_bottle?/needs_relocation? as well

needs_relocation? becomes skip_relocation?, use cellar attr to indicate relocation instead of does_not_need_relocation

MacOS.can_build? becomes MacOS.has_apple_developer_tools?

Formula/cctools.rb

@@ -12,8 +12,7 @@ class Cctools < Formula
   end
 
   bottle do
-    does_not_need_relocation
-    cellar :any
+    cellar :any_skip_relocation
     sha1 "1acad163d4a245f5bd7ad2668cc87a5c9102163a" => :yosemite
     sha1 "2629465c3d063d3a108adc987bbaa910a49db5f4" => :mavericks
     sha1 "b5ccf7ea27f82e7eb8aeed1e327079c8a07434fb" => :mountain_lion