Add support for Optional parameter to build definitions

Build definitions can now specify Optional.  If Optional is not
specified the build definition will be followed by a default definition
for all outputs.

Change-Id: Iada83b2ef32bfb599b3cc580263f12bc002e1cdb

blueprint/ninja_defs.go

@@ -227,6 +227,7 @@ type buildDef struct {
 	Implicits []*ninjaString
 	OrderOnly []*ninjaString
 	Args      map[Variable]*ninjaString
+	Optional  bool
 }
 
 func parseBuildParams(scope scope, params *BuildParams) (*buildDef,
@@ -327,6 +328,10 @@ func (b *buildDef) WriteTo(nw *ninjaWriter, pkgNames map[*PackageContext]string)
 		}
 	}
 
+	if !b.Optional {
+		nw.Default(outputs...)
+	}
+
 	return nil
 }