#v+
# In GNU, "/usr" is a four-letter word.
NATIVE_SYSTEM_HEADER_DIR = /include
#v-
I think that just changing NATIVE_SYSTEM_HEADER_DIR to:
NATIVE_SYSTEM_HEADER_DIR = $(prefix)/include
would fix all problems, could someone with a GNU/Hurd system check
that? It would kinda break stuff for people who install gcc in
{/usr,/}local though...