Bug fixes:
----------

* write all 'about' metadata fields out, not just select few  #2488
* fix lists getting nested during merging of configs, leading to TypeErrors  #2494
* make always_include_files act on host_prefix, not build_prefix  #2497
* warn users when script_env passes env vars through #2502
* fix build string pyXY being just pyX when input didn't have full python version  #2504