configure.ac: don't use python2 values for python3
configure.ac: don't use python2 values for python3

unset more internal python-related variables, so that the test for runtime-python >=3 does not reuse *any* value from test for build-python >= 2.5

Change-Id: I508292713d582476db70fcd2b05ce0abb300f38a

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core