Fix Makefile to respect CC

[dyeske]

[stappersg]

Please, have a commit message like

[build] Respect $CC

On BSD systems is `gcc` not the default compiler.
But `$CC` is honored.

(Visit https://github.com/ipxe/ipxe/commits for many goods examples of commit messages, note that they are all multi line.)

To accomplish that:

git switch respect_cc_v3
git commit --amend
# you are now in $EDITOR
# edit the commit message
# exit $EDITOR
git push

[dyeske]

The gitlab gui automatically made a merge commit onto my branch. This messes up the merge? I need to redo it on a fresh branch with a single commit that only contains my changes with the correct commit message? Thank you for your patience.

[stappersg]

Good to see there is now #723.

Consider to be extra carefull with the "gitlab gui".

No, I have no idea what a "gitlab gui" is.