Don't follow symlinks when hunting for strings

When we're assessing whether a bottle is relocatable, we shouldn't have
to descend into symlink paths we encounter. This is supposed to be the
default behavior but it doesn't appear to be (perhaps because we pass a
symlink to the keg on the command line?).

All of the switches that control this behavior differ between BSD and
GNU grep, so sniff the grep flavor first.

Library/Homebrew/keg_relocate.rb

@@ -97,7 +97,12 @@ class Keg
   end
 
   def each_unique_file_matching(string)
-    Utils.popen_read("/usr/bin/fgrep", "-lr", string, to_s) do |io|
+    bsd = `/usr/bin/fgrep -V`.include?("BSD grep")
+    grep_args = "-lr"
+    # Don't recurse into symlinks; the man page says this is the default, but
+    # it's wrong.
+    grep_args += "O" if bsd
+    Utils.popen_read("/usr/bin/fgrep", grep_args, string, to_s) do |io|
       hardlinks = Set.new
 
       until io.eof?