doxygen build failing with python confusion

[Greg Troxel <gdt%lexort.com@localhost>]

doxygen is failing with:

  [ 15%] Generating ../generated_src/configvalues.h
  cd /tmp/work/devel/doxygen/work/doxygen-1.9.6/cmake-pkgsrc-build/src && /usr/pkg/bin/python2.7 /tmp/work/devel/doxygen/work/doxygen-1.9.6/src/configgen.py -maph /tmp/work/devel/doxygen/work/doxygen-1.9.6/src/config.xml > /tmp/work/devel/doxygen/work/doxygen-1.9.6/cmake-pkgsrc-build/generated_src/configvalues.h

  No module named pyexpat

and the bug is that cmake is finding 2.7 (which is installed) instead of
the python3.10 which is the chosen version from including python for
build.  I am not particularly good at at cmake and out of time today, so
I'm mentioning it here in case any one knows how to fix it easily.
Probably it's adding -Dpython_I_want_is_3.10 or somethig to the cmake
invocation :-)