Avoid unnecessarily overwriting Makefile variables

[slicer69]

The Makefile in its original form sets certain variables, like PREFIX, in a way which will override the user's (or build environment's) existing settings. Switching the "=" to the "?=" symbol allows the host environment to set things like the compiler and PREFIX automatically, without needing to patch the Makefile.

This will make porting to other platforms like FreeBSD easier.

[leahneukirchen]

Duplicate of #20, also see #20 (comment)

[eli-schwartz]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=253086#c7

The author of this PR is trying to update FreeBSD's port (from 2017) of nq to a personal fork. Classified as a "new" port.

When pointed out that there is already that very port, slicer69 responded:

That is really weird. I did a search for nq on FreshPorts the other day before I started work on the port and got no results. Guess it's already ported.

Which seems... unlikely.

I would be very concerned about the rest of the comment:

The reason I forked upstream is the maintainer doesn't seem to have done any work on nq in the past four years and has rejected any porting/documentation pull requests. It's less about getting the new Makefile in place and just gaining a chance to keep working on it.

Bottom line is I plan to continue the source fork whether it's useful for the FreeBSD port or not, so we might as well use it.

So apparently now rejecting env FOO=bar make as equivalent to make FOO=bar constitutes "has rejected any porting/documentation pull requests" and you now have a hostile fork trying to push its way into FreeBSD.

(It has not been 4 years, it seems to have been 3 years of no one having bug reports to fix...)