default to built-in TLS without preventing other choices

tutorials/http/file-upload-single-post/Makefile

@@ -5,7 +5,8 @@ SOURCES = main.c mongoose.c       # Source code files
 CFLAGS = -W -Wall -Wextra -g -I.  # Build options
 
 # Mongoose build options. See https://mongoose.ws/documentation/#build-options
-CFLAGS_MONGOOSE ?= -DMG_TLS=MG_TLS_BUILTIN
+#CFLAGS_MONGOOSE += 
+CFLAGS_EXTRA ?= -DMG_TLS=MG_TLS_BUILTIN
 
 ifeq ($(OS),Windows_NT)   # Windows settings. Assume MinGW compiler. To use VC: make CC=cl CFLAGS=/MD OUT=/Feprog.exe
   PROG ?= example.exe           # Use .exe suffix for the binary