The "if-present" clause of AC_ARG_ENABLE() sets enable_{lzma,zstd},
so we don't need to set it. The "if-not-present" clause needs to set
enable_{lzma,zstd}, rather than just {lzma,zstd}.

The rest of the script only ever checks for enable_{lzma,zstd} = no,
so these changes do not actually change the behaviour of configure.

Fixes bug 32401.