Get the correct include path in pip package (apache#13452)

* add find_include_path API

* address reviewer comment

* change return type from list to string

* add unit test

* address reviewer comment

* address reviewer comment

* address reviewer comment

* address reviewer comment

* fix include path problem in pip package

* add comment

* fix lint error

* address reviewer comment

* address reviewer comment

python/mxnet/libinfo.py

@@ -96,10 +96,18 @@ def find_include_path():
             logging.warning("MXNET_INCLUDE_PATH '%s' doesn't exist", incl_from_env)
 
     curr_path = os.path.dirname(os.path.abspath(os.path.expanduser(__file__)))
-    incl_path = os.path.join(curr_path, '../../include/')
-    if not os.path.isdir(incl_path):
-        raise RuntimeError('Cannot find the MXNet include path.\n')
-    return incl_path
+    # include path in pip package
+    pip_incl_path = os.path.join(curr_path, 'include/')
+    if os.path.isdir(pip_incl_path):
+        return pip_incl_path
+    else:
+        # include path if build from source
+        src_incl_path = os.path.join(curr_path, '../../include/')
+        if os.path.isdir(src_incl_path):
+            return src_incl_path
+        else:
+            raise RuntimeError('Cannot find the MXNet include path in either ' + pip_incl_path +
+                               ' or ' + src_incl_path + '\n')
 
 
 # current version