build: fix secp256k1 arm64 build

- Adds CFLAGS to make command for secp256k1 build when building for
  iPhoneOS(arm64) as the configure script ignores the SDK parameter

Change-Id: Id57933d8776e7c348a0c37bae508fe61161e4151

contrib/src/secp256k1/rules.mak

@@ -18,8 +18,15 @@ secp256k1: secp256k1-$(SECP256K1_VERSION).tar.gz
 	$(UNPACK)
 	$(MOVE)
 
+ifeq ($(IOS_TARGET_PLATFORM),iPhoneOS)
+SECP256K1_CFLAGS := -USECP256K1_BUILD $(CFLAGS)
+else
+SECP256K1_CFLAGS := -USECP256K1_BUILD
+endif
+
 .secp256k1: secp256k1 .sum-secp256k1
 	$(RECONF)
 	cd $< && $(HOSTVARS) ./configure $(HOSTCONF) --disable-tests --disable-exhaustive-tests
-	cd $< && $(MAKE) CFLAGS+="-USECP256K1_BUILD" install
+	cd $< && $(MAKE) CFLAGS+='$(SECP256K1_CFLAGS)' install
+
 	touch $@