Support for RELAX NG used to be enabled together with XML Schema support
(--with-schemas). Now there's a separate option and a new feature macro
LIBXML_RELAXNG_ENABLED.
This handles static linking properly and avoids overlinking.
Change the --with-readline default to no as it was before and raise an
error if libreadline couldn't be found.
Do the same for libhistory.
libxml2 has limited support for reading and writing compressed data
with the help of zlib and liblzma which used to be enabled by default.
This only works for files read from the file system and never worked
with memory buffers. My guess is that this feature is virtually unused.
In light of the recently discovered xz backdoor, it's a good time to
disable these features by default to reduce attack surface and prepare
for eventual removal.
If --with-legacy is passed to the Autotools build, compression will
be enabled by default as before.
