Build scripts minor reformatting and better help messages.

* Rearrange Makefile wrapper so that 'make help' is much more readable.
* Added USE_CPP11, USE_LIBCPLUSPLUS, and EXTRA_CPP_ARGS (copied from
  OIIO's build scripts).
* Some minor reformatting and movement to make it align to similar constructs
  in the OIIO Makefile and CMakeLists.txt (makes it easier to diff and port
  improvements in the build system from one project to another).