build: Add optional pthread flag when detecting Crypto++

This commit also includes a small fix to support Python3

Change-Id: Ifa0c547bd872ab749745b5bdc778d4b3269bb87e
Refs: #1590
diff --git a/docs/conf.py b/docs/conf.py
index 8131fcb..3fe24dd 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -37,13 +37,11 @@
     try:
         __import__(extension)
         extensions.append(extension)
-    except ImportError, e:
-        print e
+    except ImportError:
         sys.stderr.write("Extension '%s' in not available. "
                          "Some documentation may not build correctly.\n" % extension)
         sys.stderr.write("To install, use \n"
                          "  sudo pip install %s\n" % extension.replace('.', '-'))
-        pass
 
 addExtensionIfExists('sphinxcontrib.doxylink')