ctypes: depend explicitly on ctypes.foreign

In ctypes < 0.21.0, the ctypes and ctypes.foreign libraries were
installed in the same directory, so depending on one would make the
other one visible. ctypes 0.21.0 installs them in different
directories so this makes it an error. tgls actually uses
ctypes.foreign so this adds the dependency.

Closes #29.

CHANGES.md

@@ -1,5 +1,6 @@
 
-
+* Fix build system. Explicitely depend on `ctypes.foreign`. 
+  Thanks to Etienne Millon for the patch (#29).
 * Fix `Gl.debug_message_callback` raising `Ffi_stubs.CallToExpiredClosure`. 
   Thanks to Edwin Török for the report and the patch (#6).
 

_tags

@@ -6,7 +6,7 @@ true: thread, bin_annot, safe_string
 <support/**> : package(xmlm)
 
 <src> : include
-<src/tgl{3,4,es2,es3}.{ml,mli}> : package(ctypes)
+<src/tgl{3,4,es2,es3}.{ml,mli}> : package(ctypes), package(ctypes.foreign)
 <src/tgl{3,4,es2,es3}.{cma,cmxa,cmxs}> : custom, thread, \
   package(ctypes), package(ctypes.foreign)
 <src/tgl{3,4,es2,es3}_top.*> : package(compiler-libs.toplevel)