configure: don't enable TLS when --without-* flags are used

configure: don't enable TLS when --without-* flags are used

Previously specifying `--without-gnutls` would unexpectedly attempt to
compile with GnuTLS, effectively interpreting this as
`--with-gnutls`. This caused a significant amount of confusion when
`libcurl` was built with SSL disabled since GnuTLS wasn't present.

68d89f24 dropped the `--without-*` options from the configure help, but
`AC_ARG_WITH` still defines these flags automatically. As
https://www.gnu.org/software/autoconf/manual/autoconf-2.60/html_node/External-Software.html
describes, the `action-if-given` is called when the user specifies
`--with-*` or `--without-*` options.

To prevent this confusion, we make the `--without` flag do the right
thing by ignoring the value if it set to "no".

Closes #7994