Don't build mx.DateTime support if the module can't be imported

Previously we only checked for the existence of the include files, but this
doesn't imply the presence of the module. Particularly true in restricted
environments such as virtualenv.

Closes ticket #53.

NEWS

@@ -1,3 +1,10 @@
+What's new in psycopg 2.4.2
+---------------------------
+
+  - Don't build mx.DateTime support if the module can't be imported
+    (ticket #53).
+
+
 What's new in psycopg 2.4.1
 ---------------------------
 

setup.py

@@ -450,12 +450,18 @@ if parser.has_option('build_ext', 'mx_include_dir'):
 else:
     mxincludedir = os.path.join(get_python_inc(plat_specific=1), "mx")
 if os.path.exists(mxincludedir):
-    include_dirs.append(mxincludedir)
-    define_macros.append(('HAVE_MXDATETIME','1'))
-    sources.append('adapter_mxdatetime.c')
-    depends.extend(['adapter_mxdatetime.h', 'typecast_mxdatetime.c'])
-    have_mxdatetime = True
-    version_flags.append('mx')
+    # Check if mx.datetime is importable at all: see ticket #53
+    try:
+        import mx.DateTime
+    except ImportError:
+        pass
+    else:
+        include_dirs.append(mxincludedir)
+        define_macros.append(('HAVE_MXDATETIME','1'))
+        sources.append('adapter_mxdatetime.c')
+        depends.extend(['adapter_mxdatetime.h', 'typecast_mxdatetime.c'])
+        have_mxdatetime = True
+        version_flags.append('mx')
 
 # now decide which package will be the default for date/time typecasts
 if have_pydatetime and (use_pydatetime or not have_mxdatetime):