ruby: don't build documentation when building for host either

We don't use it, so it only slows down the build.

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

package/ruby/ruby.mk

@@ -12,6 +12,8 @@ RUBY_DEPENDENCIES = host-ruby
 RUBY_MAKE_ENV = $(TARGET_MAKE_ENV)
 RUBY_CONF_OPT = --disable-install-doc
 
+HOST_RUBY_CONF_OPT = --disable-install-doc
+
 # Force optionals to build before we do
 ifeq ($(BR2_PACKAGE_BERKELEYDB),y)
 	RUBY_DEPENDENCIES += berkeleydb