doctor: Mono-specific warning for pkg-config

Mono installs its own pkg-config symlink into /usr/bin/pkg-config,
which breaks non-Mono builds. Provide a specific warning.

Closes Homebrew/homebrew#12859.

1 files changed, 9 insertions, 1 deletions

diff --git a/Library/Homebrew/cmd/doctor.rb b/Library/Homebrew/cmd/doctor.rb
index 277ce858f..11eabe281 100644
--- a/Library/Homebrew/cmd/doctor.rb
+++ b/Library/Homebrew/cmd/doctor.rb
@@ -463,7 +463,15 @@ def check_which_pkg_config
   binary = which 'pkg-config'
   return if binary.nil?
 
-  unless binary.to_s == "#{HOMEBREW_PREFIX}/bin/pkg-config" then <<-EOS.undent
+  mono_config = Pathname.new("/usr/bin/pkg-config")
+  if mono_config.exist? && mono_config.realpath.to_s.include?("Mono.framework") then <<-EOS.undent
+    You have a non-Homebrew 'pkg-config' in your PATH:
+      /usr/bin/pkg-config => #{mono_config.realpath}
+
+    This was most likely created by the Mono installer. `./configure` may
+    have problems finding brew-installed packages using this other pkg-config.
+    EOS
+  elsif binary.to_s != "#{HOMEBREW_PREFIX}/bin/pkg-config" then <<-EOS.undent
     You have a non-Homebrew 'pkg-config' in your PATH:
       #{binary}