Merge branch 'jh/gitweb-caching' (early part)

* 'jh/gitweb-caching' (early part):
  gitweb: Add optional extra parameter to die_error, for extended explanation
  gitweb: add a "string" variant of print_sort_th
  gitweb: add a "string" variant of print_local_time
  gitweb: Check that $site_header etc. are defined before using them
  gitweb: Makefile improvements
  gitweb: Load checking
  gitweb: Make running t9501 test with '--debug' reliable and usable

diff --cc Makefile
Simple merge