boost-fallback: only check for boost if really needed

[pseiderer]

• only check for boost if really needed (in case of legacy c++ standard < c++17)

Signed-off-by: Peter Seiderer ps.report@gmx.net

Notes:

• fixes the following buildroot configure/compile failure in case
  a only partly available boost is detected (but not needed as a
  c++17 capable compiler is available):

  -- Found Boost: .../host/x86_64-buildroot-linux-uclibc/sysroot/usr/include (found version "1.78.0") found components: thread chrono missing components: date_time atomic
  CMake Error at .../build/log4cxx-0.12.0/boost-fallback-compile-tests/CMakeFiles/CMakeTmp/CMakeLists.txt:19 (add_executable):
  Target "cmTC_aac37" links to target "Boost::date_time" but the target was
  not found. Perhaps a find_package() call is missing for an IMPORTED
  target, or an ALIAS target is missing?

  CMake Error at .../build/log4cxx-0.12.0/boost-fallback-compile-tests/CMakeFiles/CMakeTmp/CMakeLists.txt:19 (add_executable):
  Target "cmTC_aac37" links to target "Boost::atomic" but the target was not
  found. Perhaps a find_package() call is missing for an IMPORTED target, or
  an ALIAS target is missing?

  See http://lists.busybox.net/pipermail/buildroot/2022-January/635042.html

[rm5248]

I'm surprised that it failed - I thought that the try_compile tests would not cause it to completely fail. Maybe I just don't understand enough about how it works.

[rm5248]

I'm looking at an issue(LOGCXX-551) that has come up due to this fix - how did this situation happen in the first place? This looks a lot like the boost installation is very broken, as I've been unable to reproduce the issue, and the cmake FindBoost should properly account for the transitive boost dependencies.

[rm5248]

Wait, I've figured it out - your build is using cmake 3.16, which is not smart enough to know about boost 1.78 and thus does not take in the transitive dependencies required.