Automatically include modules with callback attributes in
erl_first_files.

* consolidate options
 * add support for building executables

Before copying the .erl files to the .eunit directory ensure that they
are deleted if they already exist. This prevents EACCES errors to happen
when trying to re-copy .erl files that are read-only.

a) make_symlink throws "not supported" on OS < Vista (e.g. 2003)
b) tarball creation stores windows symlinks as absolute paths
   instead of relative, causing service to fail to start after upgrade
   is made permanent.

Error report killing inet_gethost_native after eunit

Avoid error reports like this one:

  prompt$ rebar eunit
  ==> dummy (eunit)
    Test passed.

  =ERROR REPORT==== 29-Dec-2011::23:22:11 ===
  ** Generic server inet_gethost_native_sup terminating
  ** Last message in was {'EXIT',<0.62.0>,killed}
  ** When Server state == {state,inet_gethost_native,undefined,<0.62.0>,
                                 {local,inet_gethost_native_sup}}
  ** Reason for termination ==
  ** killed

Previously, it could happen if an eunit test did something that
required a name lookup, like in this example:

  -module(dummy_tests).
  -include_lib("eunit/include/eunit.hrl").

  x_test() ->
      {ok, _Hostent} = inet:gethostbyname(localhost).

The inet_gethost_native is a process, started on demand, under a
supervisor_bridge under the kernel_sup, but it is not a gen_server
process or the like, so it has nothing in '$ancestors' in its process
dictionary to indicate it is part of kernel.