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: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9016)

Configure

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