Verbose messages from newer compilers confuse conf.sh

conf.sh checks for undefined symbols by parsing compiler output and
looking for strings of capital letters and underscores. Modern gcc
produces suggestions for replacement variables too, for example:

error: '_SC_CLOCKRES_MIN' undeclared here (not in a function); did you mean _POSIX_CLOCKRES_MIN?
 _SC_CLOCKRES_MIN,
 ^~~~~~~~~~~~~~~~
 _POSIX_CLOCKRES_MIN

This causes good variables to be excluded along with bad, causing differences
between the builtin and system getconf commands.

src/lib/libast/comp/conf.sh

@@ -817,7 +817,8 @@ unsigned int conf[] = {
 		[ -f $tmp.1.e ] || cp $tmp.e $tmp.1.e
 		snl='\
 '
-		sed "s/[^_ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789][^_ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789]*/${snl}/g" $tmp.e |
+		grep 'error:' $tmp.e | sed "s/did you mean.*//" |
+		sed "s/[^_ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789][^_ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789]*/${snl}/g" |
 		grep '^[_ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz][_ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789]*$' |
 		sort -u > $tmp.n
 		cmp -s $tmp.n $tmp.p && break