Rebuild manpage only if needed.

auto/install

diff --git a/auto/install b/auto/install
index 6b5a120..6cf217e 100644
--- a/auto/install
+++ b/auto/install
@@ -74,7 +74,9 @@
 
 cat << END                                                    >> $NGX_MAKEFILE
 
-manpage:
+manpage:	$NGX_OBJS/nginx.8
+
+$NGX_OBJS/nginx.8:	man/nginx.8 $NGX_AUTO_CONFIG_H
 	sed -e "s|%%PREFIX%%|$NGX_PREFIX|" \\
 		-e "s|%%PID_PATH%%|$NGX_PID_PATH|" \\
 		-e "s|%%CONF_PATH%%|$NGX_CONF_PATH|" \\