Adding Makefile to the license tool.

main.go

@@ -219,7 +219,7 @@ func licenseHeader(path string, tmpl *template.Template, data *copyrightData) ([
 		lic, err = prefix(tmpl, data, "/**", " * ", " */")
 	case ".cc", ".cpp", ".cs", ".go", ".hh", ".hpp", ".java", ".m", ".mm", ".proto", ".rs", ".scala", ".swift", ".dart", ".groovy", ".kt", ".kts", ".v", ".sv":
 		lic, err = prefix(tmpl, data, "", "// ", "")
-	case ".py", ".sh", ".yaml", ".yml", ".dockerfile", "dockerfile", ".rb", "gemfile", ".tcl", ".bzl":
+	case ".py", ".sh", ".yaml", ".yml", ".dockerfile", "dockerfile", ".rb", "gemfile", ".tcl", ".bzl", "makefile" :
 		lic, err = prefix(tmpl, data, "", "# ", "")
 	case ".el", ".lisp":
 		lic, err = prefix(tmpl, data, "", ";; ", "")

testdata/expected/Makefile

@@ -0,0 +1,16 @@
+# Copyright 2018 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+hellowword:
+	@echo Hello world 

testdata/initial/Makefile

@@ -0,0 +1,2 @@
+hellowword:
+	@echo Hello world