If the local build.config has changes compared to upstream's
master branch, and $(ERLANG_MK_COMMIT) also brings changes to this
file, copying the local file to the checkout before switching to
$(ERLANG_MK_COMMIT) caused git-checkout(1) to abort with:

    error: Your local changes to the following files would be overwritten by checkout:
    	build.config
    Please, commit your changes or stash them before you can switch branches.
    Aborting

While here, fix `core-upgrade-custom-repo` test case: without switching
back the alternate erlang.mk repository to its master branch, the clone
always got the test-copyright branch by default, making the
ERLANG_MK_COMMIT check ineffective.

Main test Makefile expects "c_src" but for convenience I also
allow "c-src" and use that one as the prefix for the tests.

Pushing this now so I can figure out Windows.

Initially submitted by Daniel White.

The previous code did:
    if `<code that prints nothing on stdout>`; then ...

So the test failed but not for the good reason. The forked shell
printed:
/bin/sh: gmake[2]:: not found

Now, both "Run 'make ...'" tests are simplified and do not rely on
backticks and test(1) anymore. Either the output is inspected or the
exit code is checked.

Per request: https://github.com/ninenines/erlang.mk/issues/300

This commit also fixes issues with packages which have a different
name than the application name.