preprocessor/8505: cross cpp0 use PREFIX_INCLUDE_DIR=$prefix/include

[Zack Weinberg]

[http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=8505]

My apologies for letting this go for so long.

I am not expert in the domain of cross compilers, and I can think of
plausible reasons both to have the PREFIX_INCLUDE_DIR searched and
not to have it searched.  It would be consistent with what is done
for other directories, like LOCAL_INCLUDE_DIR, not to search it;
however, it appears that this directory has been searched in cross
compilers since forever, so people might be depending on the existing
behavior.

I would appreciate opinions.  Dan, I'm cc:ing you because you've had
your fingers in this area recently.

zw