Fix AC_PATH_PROG() calls to specify default.

Fix AC_PATH_PROG() calls to specify 'false' as the default, so that if
the configure script fails to find a program, the false program is
instead called, and an error occurs.  Prior to this fix, if xsltproc
could not be found, make would not report an error due to the leading -o
in the xsltproc invocation.

Reported by David Reiss.