default to built-in TLS without preventing other choices

tutorials/http/http-server/Makefile

@@ -8,7 +8,7 @@ CFLAGS = -W -Wall -Wextra -g -I.  # Build options
 # Mongoose build options. See https://mongoose.ws/documentation/#build-options
 CFLAGS_MONGOOSE += -DMG_HTTP_DIRLIST_TIME_FMT="%Y/%m/%d %H:%M:%S"
 CFLAGS_MONGOOSE += -DMG_ENABLE_LINES=1 -DMG_ENABLE_IPV6=1 -DMG_ENABLE_SSI=1
-CFLAGS_MONGOOSE += -DMG_TLS=MG_TLS_BUILTIN
+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