[Buildroot] QMAKESPEC has not been set

[rfinkelstein at rtsys.fr]

Hello, I am using Buildroot 2016.02.

I'm trying to build the package python-pyqt5, to do so the package 
python-sip 4.16.7 is selected,

When building the python-sip I get the error:
"QMAKESPEC has not been set, so configuration cannot be deduced."

If I create a environment variable QMAKESPEC,and export it, when I run 
only the command that generated the error in the terminal, it works, and 
go through without showing this error, but when I try to do the full 
build the error keeps showing.

Any ideia why? and maybe how to fix?