tools/flex: override SUBDIRS variable with Makefile

Instead of editing the SUBDIRS variable with a patch,
it can be overriden at the end of the command line when invoking Make.

Skip the build for po files as well, which is causing a conflict
depending on the version of gettext detected, and macros present.

Signed-off-by: Michael Pratt <mcpratt@pm.me>

tools/flex/Makefile

@@ -21,13 +21,17 @@ include $(INCLUDE_DIR)/host-build.mk
 
 HOST_CONFIGURE_ARGS += --disable-shared
 
+define Host/Compile
+	$(call Host/Compile/Default,SUBDIRS='src tools')
+endef
+
 define Host/Install
-	$(call Host/Install/Default)
+	$(call Host/Compile/Default,install SUBDIRS='src tools')
 	$(LN) flex $(STAGING_DIR_HOST)/bin/lex
 endef
 
 define Host/Uninstall
-	-$(call Host/Compile/Default,uninstall)
+	-$(call Host/Compile/Default,uninstall SUBDIRS='src tools')
 endef
 
 define Host/Clean