Make python3 only build in fluid only

paddle/scripts/paddle_build.sh

@@ -90,6 +90,7 @@ function cmake_gen() {
                 PYTHON_FLAGS="-DPYTHON_EXECUTABLE:FILEPATH=/Library/Frameworks/Python.framework/Versions/3.5/bin/python3
             -DPYTHON_INCLUDE_DIR:PATH=/Library/Frameworks/Python.framework/Versions/3.5/include/python3.5m/
             -DPYTHON_LIBRARY:FILEPATH=/Library/Frameworks/Python.framework/Versions/3.5/lib/libpython3.5m.dylib"
+                WITH_FLUID_ONLY=${WITH_FLUID_ONLY:-ON}
             else
                 exit 1
             fi