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`.