Makefile: Add ZIG variable and build target #220

The ZIG variable helps to test ncdu with different zig installations,
and it allows Gentoo to inject the zig version that should be used to
build zig into the Makefile.

Also add a phony 'build' target as first target to the Makefile so
that it becomes the default target. This allows the Gentoo package to
use the default src_compile() function.

See also https://bugs.gentoo.org/900547

Pulled. Not having 'doc' as part of the default target was kind of intentional, as I'm sure not everyone has pod2man installed and not everyone is interested in the man page. But ncdu.1 is included in the tarball and with the right the timestamps that should ensure it won't be built anyway. Peolpe can always just run zig build directly, too, so whatever.

Thanks for your work on Gentoo, much appreciated!