Misc build tweaks/updates from Nelson

* merges:
  Document why the duplicate traversal is needed
  Revert "Remove duplicate subdirs"
  Make info be generated in buildidr
  Remove duplicate subdirs
  Improve liblzma pkg-config detection
  Improve FFI pkg-config detection
  Add new pkg-config macro SXE_PC_CHECK_EXISTS
  Fix pkg-config autoconf macros
  Automatically paths in site prefix to PKG_CONFIG_PATH