allow tests to run even without execinfo.h
diff --git a/OpenSSL/test/util.py b/OpenSSL/test/util.py
index 011e7da..4e4d812 100644
--- a/OpenSSL/test/util.py
+++ b/OpenSSL/test/util.py
@@ -17,7 +17,11 @@
 from OpenSSL._util import exception_from_error_queue
 from OpenSSL.crypto import Error
 
-import memdbg
+try:
+    import memdbg
+except Exception:
+    class _memdbg(object): heap = None
+    memdbg = _memdbg()
 
 from OpenSSL._util import ffi, lib, byte_string as b
 
diff --git a/runtests.py b/runtests.py
index 2ec425b..13f5c4c 100644
--- a/runtests.py
+++ b/runtests.py
@@ -2,7 +2,10 @@
 sys.modules['ssl'] = None
 sys.modules['_hashlib'] = None
 
-import memdbg
+try:
+   import memdbg
+except Exception as e:
+   pass
 
 from twisted.scripts.trial import run
 run()