build-sys: fix out-of-tree builds without xmltoman
build-sys: fix out-of-tree builds without xmltoman

If manpages are enabled but xmltoman isn't present, out-of-tree builds fail
because it checks inside the build directory for the pre-generated manpages.

Fix this by using $srcdir when looking for files inside the source directory.

Signed-off-by: Ross Burton <cd36ec7a67dfb8d589bc66b85fd0a6cf213f3854@intel.com>
Signed-off-by: Trent Lloyd <0aa1c221ca98a492b70c16da97a2505ec24a2ae9@lloyd.id.au>

heftig/avahi-1,heftig/avahi,heftig/avahi-1,heftig/avahi,lathiat/avahi,lathiat/avahi,heftig/avahi-1,lathiat/avahi,heftig/avahi,heftig/avahi-1,lathiat/avahi,lathiat/avahi,lathiat/avahi,heftig/avahi-1,heftig/avahi-1,heftig/avahi,heftig/avahi,heftig/avahi