For Libxml2 and friends, YAZ defines YAZ_HAVE_{XML2,XSLT,EXSLT) in

For Libxml2 and friends, YAZ defines YAZ_HAVE_{XML2,XSLT,EXSLT) in
the compitation phase, but also for yaz-config --cflags output. This
allows us to have public headers with Libxml2 stuff in them . It should
also, eventually, make Libxml2 checks redundant in packages using
YAZ, such as yazproxy.