Configure: canonicalise PythonCmd on Windows

This change makes PythonCmd resolve to a canonical absolute path on
Windows, which prevents HLS getting confused (now that we have a
build-time dependency on python).

fixes #23652

m4/find_python.m4

@@ -5,6 +5,11 @@
 AC_DEFUN([FIND_PYTHON],[
     dnl Prefer the mingw64 distribution on Windows due to #17483.
     AC_PATH_PROG([PYTHON], [python3], [], [/mingw64/bin $PATH])
-    PythonCmd="$PYTHON"
+    if test "$HostOS" = "mingw32"
+    then
+      PythonCmd=$(cygpath -m "$PYTHON")
+    else
+      PythonCmd="$PYTHON"
+    fi
     AC_SUBST([PythonCmd])
 ])