Add macro -DARDUINO_LIB_DISCOVERY_PHASE during lib discovery phase (#633)

This information can be used by the core to turn off certain heavyweight headers (that don't need to be discovered anyway)

Author: facchinm

Diff for: legacy/builder/gcc_preproc_runner.go

@@ -77,6 +77,8 @@ func prepareGCCPreprocRecipeProperties(ctx *types.Context, sourceFilePath *paths
 	// to create a /dev/null.d dependency file, which won't work.
 	cmd.Args = utils.Filter(cmd.Args, func(a string) bool { return a != "-MMD" })
 
+	cmd.Args = append(cmd.Args, "-DARDUINO_LIB_DISCOVERY_PHASE")
+
 	return cmd, nil
 }