Enveigle configure to look for a2x.py as well as a2x.

It's not guaranteed that every package symlinks a2x to a2x.py; OpenBSD
does not do this, so let's just look for a2x.py as well.

configure.ac

@@ -167,7 +167,7 @@ AC_CHECK_PROG([SED],[sed],[sed],[/bin/false])
 
 dnl check for asciidoc and a2x
 AC_PATH_PROG([ASCIIDOC], [asciidoc], none)
-AC_PATH_PROG([A2X], [a2x], none)
+AC_PATH_PROGS([A2X], [a2x, a2x.py], none)
 
 AM_CONDITIONAL(USE_ASCIIDOC, test x$asciidoc = xtrue)