ruby: fix ruby/host build

  - build ruby/host extensions as static,
  - remove dependency on gmp/host

Signed-off-by: Nicolas Thill <nico@openwrt.org>

lang/ruby/Makefile

 PKG_BUILD_PARALLEL:=1
 PKG_FIXUP:=autoreconf
 
-HOST_BUILD_DEPENDS:=gmp/host
-
 include $(INCLUDE_DIR)/host-build.mk
 include $(INCLUDE_DIR)/package.mk
 include $(INCLUDE_DIR)/nls.mk
 HOST_CONFIGURE_ARGS += \
 	--disable-install-doc \
 	--disable-install-rdoc \
-	--disable-install-capi
+	--disable-install-capi \
+	--with-static-linked-ext \
 
 CONFIGURE_ARGS += \
 	--enable-shared \