`brew --env` puts HOMEBREW_SDKROOT

Library/Homebrew/cmd/--env.rb

@@ -24,6 +24,7 @@ module Homebrew extend self
       MACOSX_DEPLOYMENT_TARGET PKG_CONFIG_PATH HOMEBREW_BUILD_FROM_SOURCE
       HOMEBREW_DEBUG HOMEBREW_MAKE_JOBS HOMEBREW_VERBOSE HOMEBREW_USE_CLANG
       HOMEBREW_USE_GCC HOMEBREW_USE_LLVM HOMEBREW_SVN HOMEBREW_GIT
+      HOMEBREW_SDKROOT
       MAKE GIT CPP
       ACLOCAL_PATH OBJC PATH ].select{ |key| env[key] }
   end