[3.7] bpo-32647: Link ctypes extension with libdl. (GH-5550) (#5874)

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 <christian@python.org>
(cherry picked from commit 5bb96925)
Co-authored-by: Christian Heimes <christian@python.org>

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

+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

@@ -2005,6 +2005,10 @@ class PyBuildExt(build_ext):
             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 = []