openvpn: prefix variables with OPENVPN_

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

package/openvpn/openvpn.mk

@@ -9,14 +9,14 @@ OPENVPN_SOURCE = openvpn-$(OPENVPN_VERSION).tar.gz
 OPENVPN_SITE = http://openvpn.net/release
 OPENVPN_DEPENDENCIES = lzo openssl uclibc
 
+OPENVPN_CONF_OPT = --enable-small
+
 ifeq ($(BR2_PTHREADS_NATIVE),y)
-	THREAD_MODEL="--enable-threads=posix"
+	OPENVPN_CONF_OPT += --enable-threads=posix
 else
-	THREAD_MODEL="--enable-pthread"
+	OPENVPN_CONF_OPT += --enable-pthread
 endif
 
-OPENVPN_CONF_OPT = --enable-small $(THREAD_MODEL)
-
 $(eval $(call AUTOTARGETS,package,openvpn))
 
 $(OPENVPN_TARGET_INSTALL_TARGET):