Fix issues when use linux-sgx as cmake subdirectory (#166)

1. Remove the copy of libvmlib.a and libextlib.a to ../enclave_example,
   which doesn't exist in other project.
2. Use default SGX_SDK path to avoid "source $SGX_SDK/environment".

Signed-off-by: Junxian.Xiao <junxian.xjx@antfin.com>

product-mini/platforms/linux-sgx/enclave-sample/Makefile

@@ -170,6 +170,14 @@ Enclave/%.o: Enclave/%.cpp
 	@$(CXX) $(Enclave_Cpp_Flags) -c $< -o $@
 	@echo "CXX  <=  $<"
 
+libvmlib.a: ../build/libvmlib.a
+	@cp $< $@
+	@echo "CP $@  <=  $<"
+
+libextlib.a: ../build/libextlib.a
+	@cp $< $@
+	@echo "CP $@  <=  $<"
+
 $(Enclave_Name): Enclave/Enclave_t.o $(Enclave_Cpp_Objects) libvmlib.a libextlib.a
 	@$(CXX) $^ -o $@ $(Enclave_Link_Flags)
 	@echo "LINK =>  $@"