configure.ac: Fix test-for-empty bug

From Gaetan Nadon:

./configure: line 11662: test: !=: unary operator expected
AM_CONDITIONAL([HAVE_XSLTPROC], [test $XSLTPROC != ""])
AM_CONDITIONAL([HAVE_PUBLICAN], [test $PUBLICAN != ""])

It shows up when you do not have publican. The usual "testing for blank" bug.