Changes required to make macos build more reliable

We have found a few things that cause the build to fail on macos, depending on user's config:

• env values for CC and CXX affect it. We recommend unsetting both here https://gitlab.redox-os.org/redox-os/redox/-/blob/master/mk/prefix.mk

• it requires "makeinfo", which is part of brew texinfo package. We recommend the build fails when it is not found, not continue, and it could be documented as a pre-requisite in the book developing section.