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`, `*.C`, `*.cc`, and `*.cpp`.
plugins/c_src.mk: update CFLAGS/CXXFLAGS/LDFLAGS for OS X

* Add the following flags to LDFLAGS needed at linkage for OS X,
  and remove the following flags from CFLAGS and CPPFLAGS:
  `-arch x86_64 -flat_namespace -undefined suppress`