XXX needs Python 3.4 at least, but 2.7 was specified

[Montgomery-Smith, Stephen]

When I try to build certain ports, I get the error message "XXX needs
Python 3.4 at least, but 2.7 was specified."  Below I give an example
when I try to build the x11/mate port.  It gets this error when it tries
to build graphics/graphene as a dependency.  If I then go to
graphics/graphene and then try to build the port directly, it works just
fine.  Somehow the x11/mate port, or one of its dependencies, is saying
to use Python 3.x.  Somehow the graphene port should figure out it is
supposed to be using Python 2.x, but it seems it cannot figure this out
for itself.

Is anyone else having this problem?  I have had it a very long time,
predating the switch from defaults Python 2.x to Python 3.x.  My theory
is that this error doesn't come up if you use tools like portmaster or
poudriere, and that is why no-one has tried to fix it.  But maybe I have
something strange in my setup, and no-one else experiences it.

Example follows:

cd /usr/ports/x11/mate && make install clean

......... snipped ......

===>   gstreamer1-plugins-gl-1.14.4_2 depends on shared library:
libgraphene-1.0.so - not found
===>  graphene-1.10.0 needs Python 3.4 at least, but 2.7 was specified.
*** Error code 1

Stop.
make[6]: stopped in /usr/ports/graphics/graphene
*** Error code 1

Stop.
make[5]: stopped in /usr/ports/graphics/gstreamer1-plugins-gl
*** Error code 1

Stop.
make[4]: stopped in /usr/ports/www/webkit2-gtk3
*** Error code 1

Stop.
make[3]: stopped in /usr/ports/x11/yelp
*** Error code 1

Stop.
make[2]: stopped in /usr/ports/misc/mate-user-guide
*** Error code 1

Stop.
make[1]: stopped in /usr/ports/x11/mate-base
*** Error code 1

Stop.
make: stopped in /usr/ports/x11/mate