systemd: Optionally build manpages.

system/systemd/DEPENDS

@@ -1,4 +1,3 @@
-depends docbook-xsl
 depends kmod
 depends dbus
 depends libcap
@@ -9,3 +8,4 @@ depends gperf
 
 optional_depends "cryptsetup" "--enable-libcryptsetup" "--disable-libcryptsetup" "for Device-mapper crypto encryption"
 optional_depends "Linux-PAM" "--with-pamlibdir=/lib/security" "--disable-pam" "for PAM support"
+optional_depends "docbook-xsl" "--enable-manpages" "--disable-manpages" "for manpages"