zsh/system's $errnos empty with recent gcc

[Stephane Chazelas <stephane.chazelas@gmail.com>]

At least on Ubuntu 16.04 and Debian unstable, $errnos comes out
empty.

It's down to the generation of errnames.c. Part of that
involves calling $CPP on an awk generated file, and post-process
$CPP's output with a second awk script.

It looks like the $CPP output has changed recently. I had a very
similar issue when compiling pdksh, passing the -P (to remove
the #<n> line numbers) fixed it there and it does fix it for
zsh as well.

I don't know how portable -P is (it is for gcc anyway), but it
would make sense to use it when available here. That probably
calls for a new autoconf check. Alternatively, we could modify
the awk script to take that new gcc output into account.

This:

#include <errno.h>
XXNAMES XXENOSYS ENOSYS


going through gcc -E now gives:

[...]
# 2 "a.c"
XXNAMES XXENOSYS
# 2 "a.c" 3 4
                38


While before it would give:

[...]
# 2 "a.c" 2
XXNAMES XXENOSYS 38

With gcc -E -P, it gives:

[...]
XXNAMES XXENOSYS 38

Like before. which is the format expected by errnames2.awk.

-- 
Stephane