Configure: make 'enable-buildtest-c++' work (not be a regexp)

OpenSSL 1.1.1's Configure treats the strings in @disablables as regexps,
which means that the 'buildtest-c++' option needs a bit of escaping to
be interpreted as intended.

Reviewed-by: Matt Caswell <[email protected]>
(Merged from openssl/openssl#9016)

Author: levitte

Configure

@@ -332,7 +332,7 @@ my @disablables = (
     "autoload-config",
     "bf",
     "blake2",
-    "buildtest-c++",
+    "buildtest-c\\+\\+",
     "camellia",
     "capieng",
     "cast",