c_env plugin (defines ERTS_INCLUDE_DIR)

I tried to use the C compiler plugin with the option of using a separate Makefile in the c_src directory.  I had 2 issues with this:

1. The very useful env.mk file did not get generated or included in this case.

2. I did not see much value in recursively invoking make on a separate makefile when I could just include my rules in the top-level app makefile.

So this plugin is a stripped down version of c_src.mk that defines ERTS_INCLUDE_DIR for the top level makefile.