graphics/py-cairo: clean up

• PYTHON_CMD already contains LOCALBASE
• PYTHON is already passed to CONFIGURE_ENV
• PREFIX and *FLAGS are redundant
• Respect PREFIX != LOCALBASE while stripping [1]

PR: 202911
Submitted by: John Hein <z7dr6ut7gs@snkmail.com>
Approved by: maintainer timeout (11 months)
Approved by: portmgr blanket [1]
MFH: 2016Q3