for python use an already installed version?

["Jeremy C. Reed" <reed%reedmedia.net@localhost>]

See pkgsrc/lang/python/pyversion.mk. Why does the pkgsrc version do the 
following?

# if the default is accepted, it is first choice

 ... so skips ...

# prefer an already installed version, in order of "accepted"

I have python33 installed but it wants to build python27.

My package Makefile has:

PYTHON_FOR_BUILD_ONLY=  yes
PYTHON_VERSIONS_INCLUDE_3X=     yes
CONFIG_SHELL=           ${PYTHONBIN}
.include "../../lang/python/application.mk"

Is there a pkgsrc way so I don't needlessly build a build dependency if 
I have a good enough one already installed (for makefile)?

My workaround is to set PYTHON_VERSION_DEFAULT=33 but that is not an 
infrastructure variable.