Prevent (some) files from being compiled by core/erlc.mk

This PR extends the `erlc` core plugin by adding a new variable (i.e., `ERLC_EXCL`), which can be used to specify a list of `*.erl` files that won't be compiled by `core/erlc`.

I recently stumbled upon the need for something like this in one of the projects I am working on. In that project, one of my modules implements a parse transform, and the rest of the modules are deemed to be parse transformed by the former.

Using the `COMPILE_FIRST` did not solve the problem because `core/erlc` would still try to compile the module implementing the parse transform using the `+'{parse_transform, <module-name>'}` option (as per specified in `ERLC_OPTS`).

I don't know how common this case is. If it happens to be a rare scenario, it may not make sense to merge this into `erlc/core`.