78ca0f3f54
necessary target system headers (<fs/unicode.h> and <fs/cd9660/iso.h>)
because system headers for the host (like <sys/stat.h>) are also pulled
from there and it causes build errors if target src tree is different
from installed one on the host.
Instead, copy the necessary system headers for host's programs into
${.OBJDIR} first.
Problem reported from Geert Hendrickx, and there is no particular comment
on tech-toolchain.