add tuple_calls compiler flag for erlang 21

[gdamjan]

From http://www.erlang.org/news/123 :

• "Tuple calls" have been removed from the run-time system.

http://erlang.org/download/otp_src_21.0.readme

• OTP-14497

         For legacy code that need to use parameterized modules
         or tuple calls for some other reason, there is a new
         compiler option called tuple_calls. When this option is
         given, the compiler will generate extra code that
         emulates the old behavior for calls where the module is
         a variable.

Without this patch, mochiweb crashes when tuple calls are used

[gdamjan]

I guess the version should be bumped too perhaps?

[benoitc]

I will take care about it tonight

[benoitc]

@gdamjan it seems to not be enough to have a working version on Erlang 21 unfortunately. See the CI, I will open a new ticket for it.