Permit freeciv-manual in client less builds.

Permit freeciv-manual in client less builds.

Freeciv-web uses freeciv-manual to get some help texts Freeciv generates
from the ruleset. Having to compile the C clients would slow down the
Freeciv-web build. The build system is capable of compiling the client
source files freeciv-manual needs.

In an earlier "freeciv-manual without the client" experiment a difference in
the generated server commands and server setting output was observed. This
was interpreted as corruption caused the client not being compiled. The
experiment couldn't be replicated. The difference may have been caused by an
accidental variation in applied patches and compile time settings.

See patch #7874

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@34258 a0f10bec-cc02-0410-94fc-a9cfff90b4cd