Changes in 2.53.4

Released 2024-02-19

• Improved and simplified makefiles. Please review your build scripts
  for changed build goals, input variables and build artifact names.
  See INSTALL.md for details.
• opam build uses the makefile build system.
• Various fixes in Windows and Cygwin builds.
• Preferences "force", "prefer" and related "partial" preferences now
  work slightly differently with values "newer" and "older". Previously,
  if mtimes in both replicas were equal then always the second root
  propagated to the first root (possibly reverting user changes). It
  is now made explicit that "newer" and "older" only work when mtimes
  are different.
• Cleanups in documentation.
• Bugfixes, minor improvements, cleanups.