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).
This way, warnings from headers that come from another package (in
particular linux-headers) won't show up when they get included.
To make sure we still track dependencies, use -MD instead of -MMD.
Michael Forney