build-sys: provide fallback value for xsltproc

build-sys: provide fallback value for xsltproc

Commit ab6f56debf made the change to allow building man pages even when disabled
with ./configure --disable-manpages. This works fine, as long as xsltproc is
present. If xsltproc is not present, the command to build a man page (obviously)
fails. Unfortnately it fails with a cryptic message '-o not found', because
$(XSLTPROC) is empty. Add a fallback, to use 'xsltproc' is $(XSLTPROC) is not
defined. This way we get a nice message:

make: xsltproc: Command not found