system boost found even when building our own and other boost problems

[KrisThielemans]

USE_SYSTEM_BOOST=OFF

On Ubuntu 20.04, if boost is installed via apt, but we set USE_SYSTEM_BOOST=OFF, CMake (in dependent projects such as ISMRMD) finds the system boost anyway. This is despite us passing BOOST_ROOT etc

SIRF-SuperBuild/SuperBuild/External_Boost.cmake

Lines 80 to 84 in 4483562

	set(Boost_CMAKE_ARGS
	-DBOOST_INCLUDEDIR:PATH=${BOOST_ROOT}/include/
	-DBOOST_LIBRARYDIR:PATH=${BOOST_LIBRARY_DIR}
	-DBOOST_ROOT:PATH=${BOOST_ROOT}
	)

I think what's going on is that Ubuntu 20.04 uses Boost 1.71.0 which comes with BoostConfig.cmake. (We currently build Boost 1.65.0 which doesn't install BoostConfig.cmake). Presumably FindBoost.cmake tries to find a "config" version first before even looking at BOOST_ROOT.

The result is that some compilations could use our installed boost include files, while the linking will happen against the system version. This leads to linking problems like this

/usr/bin/ld: CMakeFiles/test_ismrmrd.dir/test_acquisitions.cpp.o: in function `__static_initialization_and_destruction_0(int, int) [clone .constprop.141]':
test_acquisitions.cpp:(.text.startup+0xa1): undefined reference to `boost::unit_test::ut_detail::auto_test_unit_registrar::auto_test_unit_registrar(boost::unit_test::basic_cstring<char const>, boost::unit_test::basic_cstring<char const>, unsigned long, boost::unit_test::decorator::collector&)'

or run-time crashes.

This was first observed by @NicoleJurjew.

USE_SYSTEM_BOOST=ON

In this case we want to pass the location on, and do

SIRF-SuperBuild/SuperBuild/External_Boost.cmake

Lines 94 to 96 in 4483562

	if (BOOST_ROOT)
	set(Boost_CMAKE_ARGS ${Boost_CMAKE_ARGS} -DBOOST_ROOT:PATH=${BOOST_ROOT})
	endif()

however, CMake allows another variable BOOSTROOT for the same purpose, as well as Boost_DIR to find a "config" version. We should therefore set those as well.