Recognize Makefile line continuations in fetchRegressOpts().

Back-patch to 9.0 (all supported versions).  This is mere
future-proofing in the context of the master branch, but commit
f6dc6dd5ba requires it of older branches.

src/tools/msvc/vcregress.pl

@@ -338,6 +338,8 @@ sub fetchRegressOpts
 	my $m = <$handle>;
 	close($handle);
 	my @opts;
+
+	$m =~ s{\\\r?\n}{}g;
 	if ($m =~ /^\s*REGRESS_OPTS\s*=(.*)/m)
 	{