zig build: adjust DESTDIR logic

now if DESTDIR is provided then the default install prefix is /usr.
otherwise the default install prefix is still zig-cache directly.

DESTDIR is prepended to the prefix to match what make install does.
d6d0bb0542Andrew Kelley committed on 7/22/2019, 2:45:59 PMparent77c2ac3
1 file changedLine totals unavailable