[3.6] bpo-32647: Link ctypes extension with libdl. (pythonGH-5550) (python#5875)

The ctypes module used to depend on indirect linking for dlopen. The shared
extension is now explicitly linked against libdl on platforms with dl.

Signed-off-by: Christian Heimes <[email protected]>
(cherry picked from commit 5bb9692)

Co-authored-by: Christian Heimes <[email protected]>

Author: miss-islington

Misc/NEWS.d/next/Library/2018-02-05-13-31-42.bpo-32647.ktmfR_.rst

@@ -0,0 +1,2 @@
+The ctypes module used to depend on indirect linking for dlopen. The shared
+extension is now explicitly linked against libdl on platforms with dl.

setup.py

@@ -2089,6 +2089,10 @@ def detect_ctypes(self, inc_dirs, lib_dirs):
             ext.libraries.append(ffi_lib)
             self.use_system_libffi = True
 
+        if sysconfig.get_config_var('HAVE_LIBDL'):
+            # for dlopen, see bpo-32647
+            ext.libraries.append('dl')
+
     def _decimal_ext(self):
         extra_compile_args = []
         undef_macros = []