xlib_libSM: only build documentation if needed

Documentation generation is slow, so only create it if requested.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/x11r7/xlib_libSM/xlib_libSM.mk

@@ -11,4 +11,9 @@ XLIB_LIBSM_INSTALL_STAGING = YES
 XLIB_LIBSM_DEPENDENCIES = xlib_libICE xlib_xtrans xproto_xproto
 XLIB_LIBSM_CONF_OPT = --without-libuuid
 
+ifneq ($(BR2_HAVE_DOCUMENTATION),y)
+# documentation generation is slow
+XLIB_LIBSM_CONF_OPT += --disable-docs
+endif
+
 $(eval $(autotools-package))