Be explicit about Boost dependency in io subsystem

[daniel-j-h]

Using find_package(Osmium REQUIRED io) and any_input/output.hpp fails at compile time if Boost is not installed:

/home/travis/build/mapbox/rewrite-exit-destination-signage/third_party/libosmium/include/osmium/io/detail/debug_output_format.hpp:44:25: fatal error: boost/crc.hpp: No such file or directory

 #include <boost/crc.hpp>

Shold FindOsmium.cmake check early for all required dependencies? As in:

find_package(Boost REQUIRED)

(Maybe even with components and a minimum version)

[joto]

This is difficult to do right, because basically all dependencies are optional, some are only needed for some tiny functionality in some corner of libosmium. In this case the dependency is only needed for the debug output, which most people probably don't need. Still, the way it is now, the code might not compile, so it is good you found this problem. We probably have to be more systematic in the way we test for dependencies and then either fail the compile or disable some functionality. Not sure how to best do this, though.