fix makefile

Author: davies

docs/api/python/Makefile

@@ -7,7 +7,7 @@ SPHINXBUILD   = sphinx-build
 PAPER         =
 BUILDDIR      = _build
 
-PYTHONPATH    = $(realpath ../../../python):$(realpath ../../../python/lib/py4j-0.8.2.1-src.zip)
+export PYTHONPATH=$(realpath ../../../python):$(realpath ../../../python/lib/py4j-0.8.2.1-src.zip)
 
 # User-friendly check for sphinx-build
 ifeq ($(shell which $(SPHINXBUILD) >/dev/null 2>&1; echo $$?), 1)