Fix in tree build of protobuf

If we are building protobuf because of an incompatible out of tree protobuf installation, then make sure that we don't accidentally use that versions includes.

Makefile

@@ -315,8 +315,8 @@ LDLIBS_SECURE += $(addprefix -l, $(LIBS_SECURE))
 ifeq ($(HAS_SYSTEM_PROTOBUF),false)
 ifeq ($(HAS_EMBEDDED_PROTOBUF),true)
 PROTOBUF_DEP = $(LIBDIR)/$(CONFIG)/protobuf/libprotobuf.a
-CPPFLAGS += -Ithird_party/protobuf/src
-LDFLAGS += -L$(LIBDIR)/$(CONFIG)/protobuf
+CPPFLAGS := -Ithird_party/protobuf/src $(CPPFLAGS)
+LDFLAGS := -L$(LIBDIR)/$(CONFIG)/protobuf $(LDFLAGS)
 PROTOC = $(BINDIR)/$(CONFIG)/protobuf/protoc
 else
 NO_PROTOBUF = true

templates/Makefile.template

@@ -332,8 +332,8 @@ LDLIBS_SECURE += $(addprefix -l, $(LIBS_SECURE))
 ifeq ($(HAS_SYSTEM_PROTOBUF),false)
 ifeq ($(HAS_EMBEDDED_PROTOBUF),true)
 PROTOBUF_DEP = $(LIBDIR)/$(CONFIG)/protobuf/libprotobuf.a
-CPPFLAGS += -Ithird_party/protobuf/src
-LDFLAGS += -L$(LIBDIR)/$(CONFIG)/protobuf
+CPPFLAGS := -Ithird_party/protobuf/src $(CPPFLAGS)
+LDFLAGS := -L$(LIBDIR)/$(CONFIG)/protobuf $(LDFLAGS)
 PROTOC = $(BINDIR)/$(CONFIG)/protobuf/protoc
 else
 NO_PROTOBUF = true