Don't fail if python isn't found.
https://savannah.nongnu.org/bugs/index.php?65945
--- CMakeLists.txt.orig	2019-12-20 07:13:39.000000000 -0600
+++ CMakeLists.txt	2024-07-03 16:19:04.000000000 -0500
@@ -26,11 +26,15 @@
 # find tools
 # find python executable, for Python3 you need a virtual env, which links python to python3 executable!
 find_package(PythonInterp 3.5)
+if(PythonInterp_FOUND)
 execute_process(COMMAND ${PYTHON_EXECUTABLE}
                         -c "import sys; print('%s.%s' % (sys.version_info.major, sys.version_info.minor))"
                         OUTPUT_VARIABLE PYTHON_VERSION
                         OUTPUT_STRIP_TRAILING_WHITESPACE)
 string(REGEX MATCH "[0-9]" PYTHON_MAJOR_VERSION ${PYTHON_VERSION})
+else()
+  set(PYTHON_MAJOR_VERSION 0)
+endif()
 
 # SWIG
 if(BUILD_TCL OR BUILD_PYTHON)