`FindBoost.cmake` can't be used with `cmake --find-package` anymore
CMake 3.9 has a regression:
$ cmake --find-package -DNAME=Boost -DCOMPILER_ID=GNU -DLANGUAGE=CXX -DMODE=EXIST
CMake Error at /usr/share/cmake/Modules/FindBoost.cmake:1057 (add_library):
Unknown CMake command "add_library".
Call Stack (most recent call first):
/usr/share/cmake/Modules/CMakeFindPackageMode.cmake:180 (find_package)
Boost not found.
CMake Error: Problem processing arguments. Aborting.
before 3.9 it was fine to "find" the boost libraries that way (also w/ -DMODE=COMPILE
and -DMODE=LINK
)