configure: error if explicitly enabled clear-memory is not supported
configure: error if explicitly enabled clear-memory is not supported

This takes 22bd8d81d8fab956085e2079bf8c29872455ce59 and
b8289b625e291bbb785ed4add31f4759241067f3 into account,
but still makes it enabled by default if it is supported
and error out in case it is unsupported and was requested.

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2