Make sure HAVE_LIBSSL is defined, even when using pkg-config

Without this, the configure script assumes and states that OpenSSL will
be used, but the code won't include support for it because there we use
the "HAVE_LIBSSL" define to test for it ("#ifdef HAVE_LIBSSL").

So define the latter when pkg-config(1) is used, too.

This fixes #257, a regression introduced by commit ad86a41ee :-/