unbreak tests, MG_TLS _must_ be defined in CFLAGS_EXTRA to allow being overwritten, as documented

tutorials/mqtt/mqtt-client/Makefile

@@ -5,7 +5,8 @@ SOURCES = main.c mongoose.c mongoose_mqtt.c packed_fs.c      # Source code files
 CFLAGS = -W -Wall -Wextra -g -I.  # Build options
 
 # Mongoose build options. See https://mongoose.ws/docs/getting-started/build-options/
-CFLAGS_MONGOOSE += -DMG_ENABLE_LINES=1 -DMG_TLS=MG_TLS_BUILTIN
+CFLAGS_EXTRA ?= -DMG_TLS=MG_TLS_BUILTIN
+CFLAGS_MONGOOSE += -DMG_ENABLE_LINES=1
 
 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