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
It's a bit ugly, but appears to work. I've not tested the 32bit arm
version, but the others run.
The static binaries are about twice as large as the ncdu 1.x
counterparts.
