Add proper Flymake support to cc-mode.el

Add proper Flymake support to cc-mode.el

Except for the important detail that it doesn't make temporary files,
the new flymake-cc backend doesn't yet behave much differently from
the old flymake-proc-legacy-flymake, i.e. it still needs a special
`check-syntax' Makefile target to provide the compiler and compilation
flags.  However, the new infrastructure created should allow less
intrusive cleverer flag guessers (yet to be written) to replace that
mechanism.

* lisp/progmodes/cc-mode.el (c-mode, c++-mode): Add to
flymake-diagnostic-functions.

* lisp/progmodes/flymake-cc.el: New file.