Prefer Debian's default Python 3 version in FindPython.cmake

Closes: #945825

debian/patches/0002-Prefer-Debian-default-version-of-Python.patch

@@ -0,0 +1,33 @@
+From: =?utf-8?q?Timo_R=C3=B6hling?= <roehling@debian.org>
+Date: Mon, 13 Sep 2021 18:59:25 +0200
+Subject: Prefer Debian default version of Python
+
+---
+ Modules/FindPython/Support.cmake | 14 ++++++++++++++
+ 1 file changed, 14 insertions(+)
+
+diff --git a/Modules/FindPython/Support.cmake b/Modules/FindPython/Support.cmake
+index 8e70e11..828e075 100644
+--- a/Modules/FindPython/Support.cmake
++++ b/Modules/FindPython/Support.cmake
+@@ -23,6 +23,20 @@ if (NOT DEFINED _${_PYTHON_PREFIX}_REQUIRED_VERSION_MAJOR)
+ endif()
+ if (_${_PYTHON_PREFIX}_REQUIRED_VERSION_MAJOR EQUAL "3")
+   set(_${_PYTHON_PREFIX}_VERSIONS 3.10 3.9 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0)
++  # Debian-specific: prefer current default version
++  execute_process(
++    COMMAND py3versions -vd
++    OUTPUT_VARIABLE _${_PYTHON_PREFIX}_DEFAULT_VERSION
++    RESULT_VARIABLE _${_PYTHON_PREFIX}_DEFAULT_RESULT
++    OUTPUT_STRIP_TRAILING_WHITESPACE
++    ERROR_QUIET
++  )
++  if(NOT _${_PYTHON_PREFIX}_DEFAULT_RESULT)
++    list(REMOVE_ITEM _${_PYTHON_PREFIX}_VERSIONS ${_${_PYTHON_PREFIX}_DEFAULT_VERSION})
++    list(INSERT _${_PYTHON_PREFIX}_VERSIONS 0 ${_${_PYTHON_PREFIX}_DEFAULT_VERSION})
++  endif()
++  unset(_${_PYTHON_PREFIX}_DEFAULT_RESULT)
++  unset(_${_PYTHON_PREFIX}_DEFAULT_VERSION)
+ elseif (_${_PYTHON_PREFIX}_REQUIRED_VERSION_MAJOR EQUAL "2")
+   set(_${_PYTHON_PREFIX}_VERSIONS 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.0)
+ else()

debian/patches/series

@@ -1 +1,2 @@
 0001-Fix-cross-compilation-with-MPI.patch
+0002-Prefer-Debian-default-version-of-Python.patch