Correct slight logic error in processing IF in build.info

This corrects a fault where the inner IF in this example was still
being acted upon:

  IF[0]
    ...whatever...
    IF[1]
      ...whatever more...
    ENDIF
  ENDIF

With this change, the inner IF is skipped over.
Reviewed-by: NMatt Caswell <matt@openssl.org>

Configure

@@ -1334,7 +1334,7 @@ if ($builder eq "unified") {
             # Info we're looking for
             qr/^\s*IF\[((?:\\.|[^\\\]])*)\]\s*$/
             => sub {
-                if (! @skip || $skip[$#skip] >= 0) {
+                if (! @skip || $skip[$#skip] > 0) {
                     push @skip, !! $1;
                 } else {
                     push @skip, -1;