brew doctor: add check for stray .pc files

Library/Homebrew/cmd/doctor.rb

@@ -89,6 +89,30 @@ def check_for_stray_static_libs
   puts
 end
 
+def check_for_stray_pcs
+  unbrewed_pcs = Dir['/usr/local/lib/pkgconfig/*.pc'].select { |f| File.file? f and not File.symlink? f }
+
+  # Package-config files which are generally OK should be added to this list,
+  # with a short description of the software they come with.
+  white_list = {
+    "fuse.pc" => "MacFuse",
+  }
+
+  bad_pcs = unbrewed_pcs.reject {|d| white_list.key? File.basename(d) }
+  return if bad_pcs.empty?
+
+  puts <<-EOS.undent
+    Unbrewed .pc files were found in /usr/local/lib/pkgconfig.
+
+    If you didn't put them there on purpose they could cause problems when
+    building Homebrew formulae, and may need to be deleted.
+
+    Unexpected .pc files:
+  EOS
+  puts *bad_pcs.collect { |f| "    #{f}" }
+  puts
+end
+
 def check_for_x11
   unless x11_installed?
     puts <<-EOS.undent
@@ -628,6 +652,7 @@ module Homebrew extend self
       check_for_macgpg2
       check_for_stray_dylibs
       check_for_stray_static_libs
+      check_for_stray_pcs
       check_gcc_versions
       check_for_other_package_managers
       check_for_x11