keep TLS options as per the TLS tutorial. Default to built-in TLS while allowing to use other stacks, as tutorials show

tutorials/websocket/websocket-client/Makefile

@@ -3,10 +3,10 @@ DELETE = rm -rf                   # Command to remove files
 OUT ?= -o $(PROG)                 # Compiler argument for output file
 SOURCES = main.c mongoose.c       # Source code files
 CFLAGS = -W -Wall -Wextra -g -I.  # Build options
-CFLAGS_MONGOOSE ?= -DMG_TLS=MG_TLS_BUILTIN
 
 # Mongoose build options. See https://mongoose.ws/documentation/#build-options
 #CFLAGS_MONGOOSE += -DMG_ENABLE_LINES
+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