Fix builds under Boost 1.90.0 (Ubuntu 26.04)

[PeterBowman]

• Boost.System no longer exists as a standalone CMake package since 1.89 (https://www.boost.org/releases/1.89.0/):

  • The stub compiled library has been removed; System has been header-only since release 1.69.
  • This may affect CMakeLists.txt files containing find_package(Boost COMPONENTS system ...). The easiest fix is to just remove system from the list of components as it's no longer required. If compatibility with Boost releases earlier than 1.69 is to be preserved, one can use find_package(Boost COMPONENTS ... OPTIONAL_COMPONENTS system).

• boost::filesystem::is_complete was replaced by is_absolute at least since 1.69, i.e. 2018 (https://www.boost.org/doc/libs/1_90_0/libs/filesystem/doc/v3.html)
• boost::filesystem::path::normalize was long deprecated and ceased to exist; the replacement code was already there, I have just activated the enclosing #if-branch (https://stackoverflow.com/a/1750710)