Rework makefile to ease packaging

[davide125]

• rename default target to qpress
• add DESTDIR support
• add clean target
• allow overriding the compiler and the build flags

[PierreLvx]

Hey 👋

Thanks for the PR!

Unfortunately, Mac OS 13 (and likely earlier versions too, but I didn't check) ships with a build of install which doesn't include the -D and -targuments.

Therefore, make errors as follows:

make install
install -D -p -m 0755 -t /usr/local/bin qpress
install: illegal option -- D
usage: install [-bCcpSsv] [-B suffix] [-f flags] [-g group] [-m mode]
               [-o owner] file1 file2
       install [-bCcpSsv] [-B suffix] [-f flags] [-g group] [-m mode]
               [-o owner] file1 ... fileN directory
       install -d [-v] [-g group] [-m mode] [-o owner] directory ...
make: *** [install] Error 64

I started this repo with Mac OS compatibility in mind, so I won't merge as-is for the moment.

[PierreLvx]

FYI: I have now added a clean target to the makefile.

On MacOS 14.6, install still does not have the required args to merge that PR.