Refactor PATH setup

- Make ORIGINAL_PATHS an array of Pathnames instead of strings
 - Append the dev tools path once in global.rb instead of build.rb

Closes Homebrew/homebrew#13075.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/build.rb

@@ -4,8 +4,6 @@
 # Rationale: Formula can use __END__, Formula can change ENV
 # Thrown exceptions are propogated back to the parent process over a pipe
 
-ORIGINAL_PATHS = ENV['PATH'].split(':').map{ |p| File.expand_path p }
-
 require 'global'
 
 at_exit do
@@ -22,10 +20,7 @@ at_exit do
     ENV.extend(HomebrewEnvExtension)
     ENV.setup_build_environment
     # we must do this or tools like pkg-config won't get found by configure scripts etc.
-    ENV.prepend 'PATH', "#{HOMEBREW_PREFIX}/bin", ':' unless ORIGINAL_PATHS.include? "#{HOMEBREW_PREFIX}/bin"
-    # this is a safety measure for Xcode 4.3 which started not installing
-    # dev tools into /usr/bin as a default
-    ENV.prepend 'PATH', MacOS.dev_tools_path, ':' unless ORIGINAL_PATHS.include? MacOS.dev_tools_path
+    ENV.prepend 'PATH', "#{HOMEBREW_PREFIX}/bin", ':' unless ORIGINAL_PATHS.include? HOMEBREW_PREFIX/'bin'
 
     # Force any future invocations of sudo to require the user's password to be
     # re-entered. This is in-case any build script call sudo. Certainly this is

Library/Homebrew/global.rb

@@ -92,7 +92,10 @@ unless ARGV.include? "--no-compat" or ENV['HOMEBREW_NO_COMPAT']
   require 'compatibility'
 end
 
-# For Xcode-only installs, we add the path of the included unix tools (like git)
-if MacOS.dev_tools_path.to_s != '/usr/bin'
+ORIGINAL_PATHS = ENV['PATH'].split(':').map{ |p| Pathname.new(File.expand_path(p)) }
+
+# Xcode-only installs place tools in non-standard locations, and we also want
+# to ensure the dev tools are in the PATH in build.rb
+unless ORIGINAL_PATHS.include? MacOS.dev_tools_path
   ENV['PATH'] = ENV['PATH'].to_s + ':' + MacOS.dev_tools_path
 end