Build: Define default options only on "default"

[a4lg]

This commit sets default options only when no related options are specified. With this, full hash (256 bucket variant) will compile just by -DTLSH_BUCKETS_256=1.

Just fixing CMakeLists.txt caused an error due to a failure to track with other changes on the version 4.0.0 on min hash (48 buckets variant) so this commit also fixes this issue.