Use gmake for FreeBSD in the rebar pre_hook configuration

rebar.config

 {erl_opts, [debug_info]}.
 
-{pre_hooks, [
-	{compile, "make -C c_src"},
-	{clean, "make -C c_src clean"}
+{pre_hooks, [{"freebsd", compile, "gmake -C c_src"},
+             {"freebsd", clean, "gmake -C c_src clean"},
+             {"(linux|darwin|solaris)", compile, "make -C c_src"},
+             {"(linux|darwin|solaris)", clean, "make -C c_src clean"}
 ]}.