global: add gtags.conf to #{etc}.

Configure global to use #{etc} as sysconfdir instead of #{prefix}/etc.

Closes Homebrew/homebrew#26868.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/global.rb

@@ -27,6 +27,7 @@ class Global < Formula
     args = %W[
       --disable-dependency-tracking
       --prefix=#{prefix}
+      --sysconfdir=#{etc}
     ]
 
     if build.with? 'exuberant-ctags'
@@ -36,6 +37,8 @@ class Global < Formula
     system "./configure", *args
     system "make install"
 
+    etc.install 'gtags.conf'
+
     # we copy these in already
     cd share/'gtags' do
       rm %w[README COPYING LICENSE INSTALL ChangeLog AUTHORS]