Considering we require this compiler, we should set a good
default for all sub-Makefiles.

Thanks to two users of the ninenines/esdl2 project,
a correct way to build NIFs on Windows has been found.

At the moment we require a specific compiler (MingW's
gcc). Maybe we can change this in the future and allow
Visual Studio and others.

Some small changes have been made to the documentation,
and the meaning of one configuration variable has changed
to not include the extension (which is decided automatically
by Erlang.mk; and configurable separately).

Enjoy!

We can write the file using the Makefile variables without going
through environment variables.

Pushing this now so I can figure out Windows.

'find', the underlying command of core_find, does not support multiple patterns
after -name.  This patch fixes multi-pattern use in c_src.mk by calling core_find
once for each pattern.

This commit implements a core_find and core_ls function that
can be used to list files recursively or not.

A few other minute changes are included and a couple hacks
removed.

Previously we only did shared libraries, now we allow executables.

Also improve greatly the handling of autopatched rebar projects
by relying once again on the rebar_utils module.