Update Autotools.gitignore (#3005)

* Update Autotools.gitignore

Add ignore-rule for Makefiles generated by configure
(directly by config.status)

* Add some descriptive comment.

Add more descriptive comment to explain why Makefile should be ignore.

Autotools.gitignore

@@ -41,3 +41,9 @@ m4/ltoptions.m4
 m4/ltsugar.m4
 m4/ltversion.m4
 m4/lt~obsolete.m4
+
+# Generated Makefile 
+# (meta build system like autotools, 
+# can automatically generate from config.status script
+# (which is called by configure script))
+Makefile