Configure: variable expand GENERATE values too

Internal documentation doesn't allow for any exception...  Therefore,
even GENERATE values should be variable expanded.

(there are historical reasons why GENERATE was excepted from variable
expansion, that aren't applicable any more)

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/15554)

Configure

@@ -2212,7 +2212,7 @@ if ($builder eq "unified") {
                          if !@skip || $skip[$#skip] > 0; },
             qr/^\s* GENERATE ${index_re} = ${value_re} $/x
             => sub { $push_to->(\%generate, $expand_variables->($+{INDEX}),
-                                undef, undef, $+{VALUE})
+                                undef, undef, $expand_variables->($+{VALUE}))
                          if !@skip || $skip[$#skip] > 0; },
             qr/^\s* (?:\#.*)? $/x => sub { },
             "OTHERWISE" => sub { die "Something wrong with this line:\n$_\nat $sourced/$f" },