getconf detection: fix compiler error msg extraction (re: 3aa01a9)

That patch didn't work for non-gcc, non-clang compilers -- at least
Solaris Studio cc. It doesn't prefix error messages with "error:".
As a result, it caused the build to fail on Solaris with native cc.

src/lib/libast/comp/conf.sh:
- Use a sed formula that should catch error messages prefixed by
  "line xx:" while still removing warnings and suggestions. This
  works on at least clang, gcc, Solaris Studio cc.

src/lib/libast/comp/conf.sh

@@ -833,7 +833,9 @@ unsigned int conf[] = {
 		[ -f $tmp.1.e ] || cp $tmp.e $tmp.1.e
 		snl='\
 '
-		grep 'error:' $tmp.e | sed "s/did you mean.*//" |
+		# Extract failing identifiers. Try to isolate compiler errors while eliminating suggestions.
+		# As of 2021, known to work on: gcc, clang, Solaris Studio cc
+		sed -n 's/[Dd]id you mean.*//; /[Ww]arning:/ d; /[Ee]rror:/ p; /[Ll]ine [0-9][0-9]*:/ p' $tmp.e |
 		sed "s/[^_ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789][^_ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789]*/${snl}/g" |
 		grep '^[_ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz][_ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789]*$' |
 		sort -u > $tmp.n