Make with -lstdc++ on BSD systems

Without linking with libstdc++ snappy_nif builds fine, but fails to
load in runtime which causes confusing errors from CouchDB side.

rebar.config

@@ -5,7 +5,8 @@
                 "c_src/snappy/*.cc"]}.
 {port_env, [
     {"CXXFLAGS", "$CXXFLAGS -DNDEBUG"},
-    {"(linux|solaris)", "LDFLAGS", "$LDFLAGS -lstdc++"}
+    {"(linux|solaris|freebsd|netbsd|openbsd|dragonfly|darwin)",
+     "LDFLAGS", "$LDFLAGS -lstdc++"}
 ]}.
 {so_name, "snappy_nif.so"}.
 {eunit_opts, [verbose]}.