Don't run certain functions on non-MACOS.

Library/Homebrew/extend/ENV.rb

@@ -201,6 +201,7 @@ module HomebrewEnvExtension
   end
 
   def macosxsdk v=MacOS.version
+    return unless MACOS
     # Sets all needed lib and include dirs to CFLAGS, CPPFLAGS, LDFLAGS.
     remove_macosxsdk
     # Allow cool style of ENV.macosxsdk 10.8 here (no "" :)

Library/Homebrew/keg_fix_install_names.rb

@@ -1,5 +1,6 @@
 class Keg
   def fix_install_names
+    return unless MACOS
     mach_o_files.each do |file|
       bad_install_names_for file do |id, bad_names|
         file.ensure_writable do

Library/Homebrew/macos.rb

@@ -227,6 +227,7 @@ module MacOS extend self
   end
 
   def mdfind id
+    return [] unless MACOS
     (@mdfind ||= {}).fetch(id.to_s) do
       @mdfind[id.to_s] = `/usr/bin/mdfind "kMDItemCFBundleIdentifier == '#{id}'"`.split("\n")
     end