Merge 'better-makeflags-recognition' and 'testing-work' into branch-1.13.2

* better-makeflags-recognition:
  make flags analysis: handle more options with args
  make flags analysis: use simpler variable names
  make flags analysis: whitespace changes
  make flags analysis: embed in a subshell
  make flags analysis: be more robust
  make flags analysis: cater to GNU make 3.83 (still unreleased as of now)
  tests: expose weaknesses in make flags analysis
  tests: improve debugging output in checks on make flags analysis
  make flags analysis: refactor, to reduce code duplication

* testing-work:
  maint: targets and recipes to simplify testing on real-world packages
  build: preparatory refactoring
  build: tiny reduction in code duplication