pkg-rebar.mk: pass C++ compiler path and options

Previously only the C compiler path and options were passed to rebar.
Erlang projects that used the C++ compiler would fail to build, which
would for example be the case with the latest version of
erlang-p1-stringprep. This fixes those errors.

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
[Thomas: adjust commit message to indicate an example of an Erlang
package that needs the C++ compiler.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>