graphics/libjxl build failure with python 3.13 also, fixed

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

I am trying to build gimp-devel, and libjxl failed.

The problem is that the upstream cmake looks for python and then uses it
to run asciidoc.  cmake finds 3.13 (because I have 2.7, 3.12, 3.13) and
asciidoc is built with 3.12 (because that's default).

  -- Found Python3: /usr/pkg/bin/python3.13 (found version "3.13.2") found components: Interpreter

  [241/249] Generating cjxl.1
  FAILED: cjxl.1 /tmp/work/graphics/libjxl/work/libjxl-0.11.1/cmake-pkgsrc-build/cjxl.1 
  cd /tmp/work/graphics/libjxl/work/libjxl-0.11.1/cmake-pkgsrc-build && /usr/pkg/bin/python3.13 /usr/pkg/bin/a2x --format manpage --destination-dir="/tmp/work/graphics/libjxl/work/libjxl-0.11.1/cmake-pkgsrc-build" /tmp/work/graphics/libjxl/work/libjxl-0.11.1/doc/man/cjxl.txt
  Traceback (most recent call last):
    File "/usr/pkg/bin/a2x", line 5, in <module>
      from asciidoc.a2x import cli
  ModuleNotFoundError: No module named 'asciidoc'
  ninja: build stopped: subcommand failed.
  *** Error code 1

CMakeFile has a horror show of second-guessing python for a2x.
Apparently this is due to a misguided choice to accomodate Windows.


I just committed a fix to remove the code that looks for python, and
things seem better.   Let me know if you think a different fix should
have been applied.