CMake: Fixup stand-alone build to not depend on removed system stub library

[nigels-com]

I had trouble building cobalt stand-alone using system-installed boost 1.89.0, apparently due to the
removal of Boost::system. This changes uses the Boost_INCLUDE_DIR variable instead.

Without this change:

$ CMAKE_PREFIX_PATH=/opt cmake .. -G Ninja -DCMAKE_CXX_COMPILER=g++-14 -DCMAKE_BUILD_TYPE=Debug
CMake Error at /opt/boost-1.89.0/lib/cmake/Boost-1.89.0/BoostConfig.cmake:141 (find_package):
  Could not find a package configuration file provided by "boost_system"
  (requested version 1.89.0) with any of the following names:

    boost_systemConfig.cmake
    boost_system-config.cmake

  Add the installation prefix of "boost_system" to CMAKE_PREFIX_PATH or set
  "boost_system_DIR" to a directory containing one of the above files.  If
  "boost_system" provides a separate development package or SDK, be sure it
  has been installed.
Call Stack (most recent call first):
  /opt/boost-1.89.0/lib/cmake/Boost-1.89.0/BoostConfig.cmake:262 (boost_find_component)
  /usr/share/cmake-3.28/Modules/FindBoost.cmake:594 (find_package)
  CMakeLists.txt:178 (find_package)

[klemens-morgenstern]

Why did you remove context ?

[nigels-com]

Oh that is not intended, will revise.

[nigels-com]

Ready for further review now.