[macos] Treat /usr/local/include as a "system" include path

https://stackoverflow.com/questions/2441047/how-do-i-set-scons-system-include-path

Author: WasabiFan

tools/build_script_generator/scons/resources/SConscript.in

@@ -138,11 +138,13 @@ env.Append(MODM_BOSSAC_OPTIONS = "{{ bossac_options }}")
 env["XPCC_SYSTEM_DESIGN"] = "$BASEPATH/modm/tools/xpcc_generator"
 %% endif
 
-
-env.AppendUnique(CPPPATH=[
 %% if is_modm and family == "darwin"
-	"/usr/local/include",
+env.AppendUnique(CPPFLAGS=[
+	"-isystem", "/usr/local/include",
+])
 %% endif
+
+env.AppendUnique(CPPPATH=[
 %% for path in include_paths | sort
     abspath("{{ path | relocate | modm.windowsify(escape_level=1) }}"),
 %% endfor