secp256k1: properly set CFLAGS

Change-Id: I2f6699f924bdc74f9a728f2141f806a7a8377e00

contrib/src/secp256k1/rules.mak

@@ -35,6 +35,6 @@ endif
 .secp256k1: secp256k1 .sum-secp256k1
 	$(RECONF)
 	cd $< && $(HOSTVARS) ./configure $(HOSTCONF) --disable-tests --disable-exhaustive-tests
-	cd $< && $(MAKE) CFLAGS+='$(SECP256K1_CFLAGS)' install
+	cd $< && CFLAGS="$(CFLAGS) $(SECP256K1_CFLAGS)" $(MAKE) install
 
 	touch $@