emacs and ctags conflict

Both emacs and ctags install an executable named 'ctags'. Both should let
the user know before trying to install the other one (rather than fail
during linking).

Closes Homebrew/homebrew#16825.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/ctags.rb

@@ -7,6 +7,9 @@ class Ctags < Formula
 
   head 'https://ctags.svn.sourceforge.net/svnroot/ctags/trunk'
 
+  conflicts_with 'emacs',
+    :because => 'Both ctags and emacs install a `ctags` executable.'
+
   depends_on :autoconf => :build if build.head?
 
   def patches

Formula/emacs.rb

@@ -6,6 +6,9 @@ class Emacs < Formula
   mirror 'http://ftp.gnu.org/pub/gnu/emacs/emacs-24.2.tar.bz2'
   sha1 '38e8fbc9573b70a123358b155cf55c274b5a56cf'
 
+  conflicts_with 'ctags',
+    :because => 'Both emacs and ctags install a `ctags` executable.'
+
   option "cocoa", "Build a Cocoa version of emacs"
   option "srgb", "Enable sRGB colors in the Cocoa version of emacs"
   option "with-x", "Include X11 support"