Set {build.library_discovery_phase} to 1 during lib discovery

Previously we used to add `-DARDUINO_LIB_DISCOVERY_PHASE` to the gcc
command line but this produced some incompatiblity with compilers using
non-standard `-d` flag instead of `-D`:

arduino#633 (comment)

Author: cmaglie

Diff for: legacy/builder/gcc_preproc_runner.go

@@ -57,6 +57,7 @@ func GCCPreprocRunnerForDiscoveringIncludes(ctx *types.Context, sourceFilePath *
 
 func prepareGCCPreprocRecipeProperties(ctx *types.Context, sourceFilePath *paths.Path, targetFilePath *paths.Path, includes paths.PathList) (*exec.Cmd, error) {
 	properties := ctx.BuildProperties.Clone()
+	properties.Set("build.library_discovery_phase", "1")
 	properties.SetPath(constants.BUILD_PROPERTIES_SOURCE_FILE, sourceFilePath)
 	properties.SetPath(constants.BUILD_PROPERTIES_PREPROCESSED_FILE_PATH, targetFilePath)
 

Diff for: legacy/builder/setup_build_properties.go

@@ -67,6 +67,7 @@ func (s *SetupBuildProperties) Run(ctx *types.Context) error {
 	buildProperties.Set("build.fqbn", ctx.FQBN.String())
 	buildProperties.Set("ide_version", ctx.ArduinoAPIVersion)
 	buildProperties.Set("runtime.os", properties.GetOSSuffix())
+	buildProperties.Set("build.library_discovery_phase", "0")
 
 	if ctx.OptimizeForDebug {
 		if buildProperties.ContainsKey("compiler.optimization_flags.debug") {