plugins/c_src.mk: use non-regex file extention detection

* The `-regex` option of `find` command to determine `SOURCES` in
  plugins/c_src.mk` is not portable; does not work on OS X.
  Rewrite with a portable form of using `find` native `-name`
  command option with `\(`, `-o`, and `\)`.

* Support file extentions of `*.c`, `*.cc`, `*.cpp`.

* Add LDFLAGS for OS X for proper NIF linking.
Fixed on https://github.com/jj1bdx/erlang.mk/commit/6923e75bd37f46e76c39fe95bbdc4dce1b3ae1fc