amiga: do not hardcode openssl/zlib into the os config [ci skip]
Enable them in `lib/makefile.amiga` and `src/makefile.amiga` instead.
This allows builds without openssl and/or zlib. E.g. with the
<https://github.com/bebbo/amiga-gcc> cross-compiler.
Reviewed-by: Daniel Stenberg
Closes #9762