Don't conditionalize these variables

DISTCLEANFILES, CLEANFILES and EXTRA_DIST should not be optional and
should always be defined, otherwise things won't work correctly.

Fixes: 6ca4b63 ("Simplfy the process of building man pages")
Signed-off-by: Wouter Verhelst <w@uter.be>

man/Makefile.am

@@ -1,9 +1,9 @@
-if MANPAGES
-man_MANS = nbd-server.1 nbd-server.5 nbd-client.8 nbd-trdump.1 nbd-trplay.1 nbdtab.5
 CLEANFILES = manpage.links manpage.refs
 DISTCLEANFILES = nbd-server.1 nbd-server.1.sgml nbd-client.8 nbd-client.8.sgml nbd-server.5 nbd-server.5.sgml nbd-trdump.1 nbd-trdump.1.sgml nbd-trplay.1 nbd-trplay.1.sgml nbdtab.5 nbdtab.5.sgml
 EXTRA_DIST = nbd-server.1.sgml.in nbd-client.8.sgml.in nbd-server.5.sgml.in nbd-trdump.1.sgml.in nbd-trplay.1.sgml.in nbdtab.5.sgml.in
 
+if MANPAGES
+man_MANS = nbd-server.1 nbd-server.5 nbd-client.8 nbd-trdump.1 nbd-trplay.1 nbdtab.5
 %: %.sgml
 	docbook2man $<
 endif