Allow empty pythonincdir option
Allow empty pythonincdir option

The include path options can be empty if the python is installed in
the standard header path.  So, configure shouldn't check its emptiness.

gittup/alsa-lib,takaswie/alsa-lib,lgirdwood/alsa-lib,zedongchen/alsa-lib,takaswie/alsa-lib,gittup/alsa-lib,takaswie/alsa-lib,Distrotech/alsa-lib,mikelima/alsa-lib,Distrotech/alsa-lib,mengdonglin/alsa-lib,lgirdwood/alsa-lib,Distrotech/alsa-lib,lgirdwood/alsa-lib,mengdonglin/alsa-lib,mikelima/alsa-lib,lgirdwood/alsa-lib,gittup/alsa-lib,mikelima/alsa-lib,tiwai/alsa-lib,mengdonglin/alsa-lib,zedongchen/alsa-lib,alsa-project/alsa-lib,alsa-project/alsa-lib,gittup/alsa-lib,tiwai/alsa-lib,alsa-project/alsa-lib,lgirdwood/alsa-lib,tiwai/alsa-lib,alsa-project/alsa-lib,mikelima/alsa-lib,Distrotech/alsa-lib,tiwai/alsa-lib,mengdonglin/alsa-lib,Distrotech/alsa-lib,mikelima/alsa-lib,zedongchen/alsa-lib,zedongchen/alsa-lib,takaswie/alsa-lib,takaswie/alsa-lib,mengdonglin/alsa-lib,zedongchen/alsa-lib