Add tests for C linkage proto generation

Author: facchinm

src/arduino.cc/builder/test/sketch_with_externC_multiline/sketch_with_externC_multiline.ino

@@ -0,0 +1,34 @@
+void setup() {
+   // put your setup code here, to run once:
+ }
+ 
+ void loop() {
+   // put your main code here, to run repeatedly:
+   test2();
+   test4();
+   test6();
+   test7();
+   test10();
+ }
+
+ extern "C" {
+   void test2() {}
+ }
+  
+ extern "C" 
+ {
+   void test4() {}
+ }
+  
+ extern    "C"    
+ 
+ {
+   void test6() {}
+ }
+
+ // this function should not have C linkage
+ void test7() {}
+
+ extern    "C"     void test10() {
+   
+ };

src/arduino.cc/builder/test/try_build_of_problematic_sketch_test.go

@@ -205,6 +205,10 @@ func TestTryBuild039(t *testing.T) {
 	tryBuildWithContext(t, ctx, "sketch12", "sketch12.ino")
 }
 
+func TestTryBuild040(t *testing.T) {
+	tryBuild(t, "sketch_with_externC_multiline", "sketch_with_externC_multiline.ino")
+}
+
 func makeDefaultContext(t *testing.T) *types.Context {
 	DownloadCoresAndToolsAndLibraries(t)