msmtp: fix static linking with openssl

Openssl depends on zlib, so we need to explicitly link with
zlib as well when statically linking.

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

package/msmtp/msmtp.mk

@@ -13,6 +13,10 @@ MSMTP_DEPENDENCIES += host-pkg-config
 ifeq ($(BR2_PACKAGE_OPENSSL),y)
 MSMTP_CONF_OPT += --with-ssl=openssl
 MSMTP_DEPENDENCIES += openssl
+ifeq ($(BR2_PREFER_STATIC_LIB),y)
+# openssl uses zlib, so we need to explicitly link with it when static
+MSMTP_CONF_ENV += LIBS=-lz
+endif
 else ifeq ($(BR2_PACKAGE_GNUTLS),y)
 MSMTP_CONF_OPT += --with-ssl=gnutls
 MSMTP_DEPENDENCIES += gnutls