Suppose the following snippet:

 $ cat test.vala
 class Foo : Object {
 }

If I generate an internal vapi file without --includedir:

 $ valac -C -vapi=foo.vapi --internal-vapi=foo-internal.vapi --header=foo.h -h foo-internal.h --library=libfoo test.vala

 $ cat foo-internal.vapi
 /* foo-internal.vapi generated by valac 0.36.11, do not modify. */

 [CCode (cheader_filename = "foo-internal.h")]
 internal class Foo : GLib.Object {
	 public Foo ();
 }

This is expected.  However, if I add --includedir to the command line:

 $ valac -C --includedir=foo --vapi=foo.vapi --internal-vapi=foo-internal.vapi --header=foo.h -h foo-internal.h --library=libfoo test.vala

 $ cat foo-internal.vapi
 /* foo-internal.vapi generated by valac 0.36.11, do not modify. */

 [CCode (cheader_filename = "foo/foo.h")] // <--
 internal class Foo : GLib.Object {
	 public Foo ();
 }

The internal vapi file refers to the public header file instead.

I believe this is a regression in 0.36.x.  It is causing FTBFS in my packages, e.g.,:
https://github.com/ueno/libkkc/pull/13