Use compiler.cpp.extra_flags instead of build.extra_flags

This prevents shadowing of the ESP32 platform flags and allows Remote.c
to be compiled together with the theme editor.

arduino/arduino-cli#846

Author: max-arnold

ats-mini/Makefile

@@ -28,7 +28,7 @@ ifdef THEME_EDITOR
 endif
 
 OPTIONS = \
-	--build-property "build.extra_flags=$(DEFINES)" \
+	--build-property "compiler.cpp.extra_flags=$(DEFINES)" \
 	--warnings all
 
 HEADERS = \

docs/source/development.md

@@ -24,7 +24,7 @@ The available options are:
 To set an option, add the `--build-property` command line argument like this:
 
 ```shell
-arduino-cli compile --build-property "build.extra_flags=-DTHEME_EDITOR -DDISABLE_HOLDOFF" --clean -e -p COM_PORT -u ats-mini
+arduino-cli compile --build-property "compiler.cpp.extra_flags=-DTHEME_EDITOR -DDISABLE_HOLDOFF" --clean -e -p COM_PORT -u ats-mini
 ```
 
 ## Using the make command