Don't call xcrun twice

Library/Homebrew/os/mac.rb

@@ -60,10 +60,9 @@ module OS
       elsif tools_in_prefix? "/"
         # probably a safe enough assumption (the unix way)
         Pathname.new "/usr/bin"
-      elsif not `/usr/bin/xcrun -find make 2>/dev/null`.empty?
+      elsif not (make_path = `/usr/bin/xcrun -find make 2>/dev/null`).empty?
         # Note that the exit status of system "xcrun foo" isn't always accurate
-        # Wherever "make" is there are the dev tools.
-        Pathname.new(`/usr/bin/xcrun -find make`.chomp).dirname
+        Pathname.new(make_path.chomp).dirname
       elsif File.exist? "#{Xcode.prefix}/usr/bin/make"
         # cc stopped existing with Xcode 4.3, there are c89 and c99 options though
         Pathname.new "#{Xcode.prefix}/usr/bin"