make : fix darwin f16c flags check (#615)

...there was no check.  ported upstream from https://github.com/zanussbaum/gpt4all.cpp/pull/2 (I dont see any clean path for upstream patches)

Makefile

@@ -71,7 +71,10 @@ endif
 #       feel free to update the Makefile for your architecture and send a pull request or issue
 ifeq ($(UNAME_M),$(filter $(UNAME_M),x86_64 i686))
 	ifeq ($(UNAME_S),Darwin)
-		CFLAGS += -mf16c
+		F16C_M := $(shell sysctl machdep.cpu.features)
+		ifneq (,$(findstring F16C,$(F16C_M)))
+		    CFLAGS += -mf16c
+		endif
 		AVX1_M := $(shell sysctl machdep.cpu.features)
 		ifneq (,$(findstring FMA,$(AVX1_M)))
 			CFLAGS += -mfma