thunderbird build problem

[Dmitry Sivachenko]

Hello!

I suggest to change
USE_PYTHON_BUILD=yes
for
USE_PYTHON_BUILD=-2.7

Without this change on my machine with python3.2 installed
ports system detects and uses python3.2, but configure script complains about 
unsupported version of python (need >2.5 but not 3.x).

Thanks.