Previously, we just added the openbsd header directories to the
search path with -idirafter to prevent openbsd headers from being
used where we already have musl headers.
However, if the system had its own version of the headers (such as
sys/tree.h), those would get preferred over the openbsd versions,
which may result in build failures due to slight incompatibilities.
To fix this, just isolate the few headers we do need into their own
directory, and include it normally (with -I/-isystem).
Since ninja 1.8.0[0] and samurai 0.2[1], output mtime is always
recorded after a job succeeds, and outputs are considered dirty if
the recorded mtime is older than any input. This means that even
if a command partially writes its output then fails (for example,
if run with stdout redirected to a file), it will still get re-run
during the next build.
This means that outputs no longer have to be written atomically
(i.e. renamed into place) in order to ensure a correct build, and
we can drop the idiom `command >$out.tmp && mv $out.tmp $out`.
[0] https://github.com/ninja-build/ninja/commit/04d886b11041bb59d01df794cce7a1e8cad2250d
[1] https://github.com/michaelforney/samurai/commit/799bc5a4719b90a1026c129b55b92c0bccf97eeb
Michael Forney