libmicrohttpd: get rid of OpenSSL dependency

libmicrohttpd: get rid of OpenSSL dependency

Depending on both, GnuTLS and OpenSSL doesn't make sense, we should
use either TLS implementation but not both. Use GnuTLS for now and
get rid of OpenSSL dependency by explicitely telling the package's
build-system to do so using CONFIGURE_ARGS.

If future uses of libmicrohttpd require OpenSSL, it probably makes
most sense to build several variants of the package, i.e.
libmicrohttpd-openssl as well as libmicrohttpd-gnutls. As with most
packages making use of any TLS implementation, these are build-time
decissions resulting in conflicting variants of the same package
which cannot be installed simultanously.

Signed-off-by: Daniel Golle <redacted>