mailsend: fix ssl variant to use opensssl

mailsend: fix ssl variant to use opensssl

As described in #28261 Not compiled with OpenSSL, the SSL variant of
the mailsend package is not actually being compiled with OpenSSL.

This is due to an upstream configure check borrowed from an ancient
version of BIND, which no longer works.

As a workaround we add `-DHAVE_OPENSSL=1` to the `TARGET_CFLAGS` when
building the SSL variant.

This results in a complaint about COPTS not being honoured correctly,
but results in `mailsend` compiled with OpenSSL (i.e. works).

Signed-off-by: Daniel F. Dickinson <redacted>