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