When compiling on FreeBSD, use gmake instead of make.

Without this patch, erlydtl compilation on FreeBSD fails with the following:

snar@mars:~/compile/tmp/erlydtl>./rebar compile
==> erlydtl (compile)
erl -noshell -pa ebin  -eval 'eunit:test("ebin",[])'  -s init stop
  There were no tests to run.
Compiled src/erlydtl_parser.yrl
Compiled src/erlydtl_library.erl
[....]
Compiled src/erlydtl_contrib_humanize.erl
src/erlydtl_beam_compiler.erl:none: undefined parse transform 'merl_transform'
ERROR: compile failed while processing /usr/home/snar/compile/tmp/erlydtl: rebar_abort