Use which method in doctor

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

Library/Homebrew/cmd/doctor.rb

@@ -457,23 +457,22 @@ def check_user_path_3
 end
 
 def check_which_pkg_config
-  binary = `/usr/bin/which pkg-config`.chomp
-  return if binary.empty?
+  binary = which 'pkg-config'
+  return if binary.nil?
 
-  unless binary == "#{HOMEBREW_PREFIX}/bin/pkg-config"
-    <<-EOS.undent
-      You have a non-brew 'pkg-config' in your PATH:
-        #{binary}
+  unless binary.to_s == "#{HOMEBREW_PREFIX}/bin/pkg-config" then <<-EOS.undent
+    You have a non-Homebrew 'pkg-config' in your PATH:
+      #{binary}
 
-      `./configure` may have problems finding brew-installed packages using
-      this other pkg-config.
+    `./configure` may have problems finding brew-installed packages using
+    this other pkg-config.
     EOS
   end
 end
 
 def check_pkg_config_paths
-  binary = `/usr/bin/which pkg-config`.chomp
-  return if binary.empty?
+  binary = which 'pkg-config'
+  return if binary.nil?
 
   pkg_config_paths = `pkg-config --variable pc_path pkg-config`.chomp.split(':')