CMake: Use import targets in FindDEAL_II_BOOST.cmake

[tamiko]

Depends on #15252 #15253

[tamiko]

@masterleinad This is the goal 😄 (this here works with all pull requests merged and with CMake 3.19 onwards).

[masterleinad]

Found Boost: /home/runner/work/dealii/boost (found suitable version "1.74.0", minimum required is "1.59") found components: iostreams serialization system thread missing components: regex chrono date_time atomic
-- Processing BOOST variables and targets
-- Processing BOOST variables and targets - Done
-- Performing Test BOOST_IOSTREAMS_USABLE
-- Performing Test BOOST_IOSTREAMS_USABLE - Failed
-- Performing Test DEAL_II_BOOST_HAS_BROKEN_HEADER_DEPRECATIONS_OK
-- Performing Test DEAL_II_BOOST_HAS_BROKEN_HEADER_DEPRECATIONS_OK - Failed
-- Test unsuccessful, define DEAL_II_BOOST_HAS_BROKEN_HEADER_DEPRECATIONS
-- 
-- Defining target: interface_boost
--     VERSION:             1.74.0
--     IMPORTED TARGETS:    Boost::iostreams;Boost::serialization;Boost::system;Boost::thread
--     copying Boost::iostreams into interface_boost ...
CMake Error at cmake/macros/macro_copy_target_properties.cmake:86 (message):
  Undefined imported target name »Boost::regex« present in interface of
  target »Boost::iostreams«.

I'm surprised that we are suddenly seeing BOOST_IOSTREAMS_USABLE failing.

[tamiko]

@masterleinad This probably comes from the fact that this particular FindBoost.cmake version forgets to also export dependent targets. Let me push a fix.

So with a full Boost installation with version 1.74 onwards everything configures as expected.

With version 1.71 we see a failure with the serialization check.

With a custom-installed version 1.74 CMake is unable to find some modules but the find module emits these targets nevertheless:

2023-05-24T08:25:15.5140159Z -- Found Boost: /home/runner/work/dealii/boost (found suitable version "1.74.0", minimum required is "1.59") found components: iostreams serialization system thread missing components: regex chrono date_time atomic

[tamiko]

Update: oops the boost serialization test does not work at all with import targets. We simply don't see it for boost 1.74 onwards because we skip this expensive check in this case.