BUILD: makefile: support WOLFSSL_CFLAGS and WOLFSSL_LDFLAGS

The CFLAGS and LDFLAGS appended by USE_WOLFSSL can now be overridden
using WOLFSSL_CFLAGS and WOLFSSL_LDFLAGS.
diff --git a/Makefile b/Makefile
index d0b3344..34f15ee 100644
--- a/Makefile
+++ b/Makefile
@@ -573,8 +573,10 @@
 endif
 
 ifneq ($(USE_OPENSSL_WOLFSSL),)
-  OPTIONS_CFLAGS   += $(if $(WOLFSSL_INC),-I$(WOLFSSL_INC) -I$(WOLFSSL_INC)/wolfssl)
-  OPTIONS_LDFLAGS  += $(if $(WOLFSSL_LIB),-L$(WOLFSSL_LIB)) -lwolfssl
+  WOLFSSL_CFLAGS    = $(if $(WOLFSSL_INC),-I$(WOLFSSL_INC) -I$(WOLFSSL_INC)/wolfssl)
+  WOLFSSL_LDFLAGS   = $(if $(WOLFSSL_LIB),-L$(WOLFSSL_LIB)) -lwolfssl
+  OPTIONS_CFLAGS   += $(WOLFSSL_CFLAGS)
+  OPTIONS_LDFLAGS  += $(WOLFSSL_LDFLAGS)
 endif
 
 ifneq ($(USE_ENGINE),)