caveats: tweak keg-only messaging.

Slim down the existing text and make it more obvious what's happening.
Note specifically how to add it to your PATH to convince people to try
that instead of `brew link --force`.

Library/Homebrew/caveats.rb

@@ -48,20 +48,23 @@ class Caveats
 
     s = "This formula is keg-only, which means it was not symlinked into #{HOMEBREW_PREFIX}."
     s << "\n\n#{f.keg_only_reason}"
-    if f.lib.directory? || f.include.directory?
-      s <<
-        <<-EOS.undent_________________________________________________________72
-
-
-        Generally there are no consequences of this for you. If you build your
-        own software and it requires this formula, you'll need to add to your
-        build variables:
+    if f.bin.directory? || f.sbin.directory?
+      s << "\nIf you need to have this software first in your PATH run:\n"
+      if f.bin.directory?
+        s << "  #{Utils::Shell.prepend_path_in_shell_profile(f.opt_bin.to_s)}\n"
+      end
+      if f.sbin.directory?
+        s << "  #{Utils::Shell.prepend_path_in_shell_profile(f.opt_sbin.to_s)}\n"
+      end
+    end
 
-        EOS
+    if f.lib.directory? || f.include.directory?
+      s << "\nFor compilers to find this software you may need to set:\n"
       s << "    LDFLAGS:  -L#{f.opt_lib}\n" if f.lib.directory?
       s << "    CPPFLAGS: -I#{f.opt_include}\n" if f.include.directory?
-
-      if which("pkg-config")
+      if which("pkg-config") &&
+         ((f.lib/"pkgconfig").directory? || (f.share/"pkgconfig").directory?)
+        s << "For pkg-config to find this software you may need to set:\n"
         s << "    PKG_CONFIG_PATH: #{f.opt_lib}/pkgconfig\n" if (f.lib/"pkgconfig").directory?
         s << "    PKG_CONFIG_PATH: #{f.opt_share}/pkgconfig\n" if (f.share/"pkgconfig").directory?
       end