Allow .cxx and .cc extensions

Author: cmaglie

Diff for: arduino/globals/globals.go

@@ -35,6 +35,8 @@ var (
 		".hpp":  empty,
 		".hh":   empty,
 		".cpp":  empty,
+		".cxx":  empty,
+		".cc":   empty,
 		".S":    empty,
 		".adoc": empty,
 		".md":   empty,
@@ -43,11 +45,15 @@ var (
 		".ipp":  empty,
 	}
 
-	// SourceFilesValidExtensions lists valid extensions for source files (no headers)
-	SourceFilesValidExtensions = map[string]struct{}{
-		".c":   empty,
-		".cpp": empty,
-		".S":   empty,
+	// SourceFilesValidExtensions lists valid extensions for source files (no headers).
+	// If a platform do not provide a compile recipe for a specific file extension, this
+	// map provides the equivalent extension to use as a fallback.
+	SourceFilesValidExtensions = map[string]string{
+		".c":   "",
+		".cpp": "",
+		".cxx": ".cpp",
+		".cc":  ".cpp",
+		".S":   "",
 	}
 
 	// HeaderFilesValidExtensions lists valid extensions for header files

Diff for: docs/platform-specification.md

@@ -172,12 +172,15 @@ These properties can be overwritten respectively with `--keys-keychain`, `--sign
 #### Recipes to compile source code
 
 We said that the Arduino development software determines a list of files to compile. Each file can be source code
-written in C (.c files), C++ (.cpp files) or Assembly (.S files). Every language is compiled using its respective
-**recipe**:
+written in C (.c files), C++ (.cpp/.cxx/.cc files) or Assembly (.S files). Every language is compiled using its
+respective **recipe**:
 
-- `recipe.c.o.pattern`: for C files
-- `recipe.cpp.o.pattern`: for CPP files
-- `recipe.S.o.pattern`: for Assembly files
+- `recipe.c.o.pattern`: for C files (.c)
+- `recipe.cpp.o.pattern`: for CPP files (.cpp/.cxx/.cc)
+- `recipe.S.o.pattern`: for Assembly files (.S)
+
+(an optional `recipe.cxx.o.pattern` and `recipe.cc.o.pattern` may be provided, if `.cxx` or `.cc` needs special
+handling, but it's not required and we do not recommend it)
 
 The recipes can be built concatenating the following automatically generated properties (for each file compiled):
 

Diff for: legacy/builder/builder_utils/utils.go

@@ -109,6 +109,9 @@ func compileFiles(ctx *types.Context, sourcePath *paths.Path, recurse bool, buil
 	queue := make(chan *paths.Path)
 	job := func(source *paths.Path) {
 		recipe := fmt.Sprintf("recipe%s.o.pattern", source.Ext())
+		if !buildProperties.ContainsKey(recipe) {
+			recipe = fmt.Sprintf("recipe%s.o.pattern", globals.SourceFilesValidExtensions[source.Ext()])
+		}
 		objectFile, err := compileFileWithRecipe(ctx, sourcePath, source, buildPath, buildProperties, includes, recipe)
 		if err != nil {
 			errorsMux.Lock()