We may need to guess `CMAKE_FIND_ROOT` when cross-compiling

[illwieckz]

When corss-compiling, CMake lines like this don't work:

find_library(LIBRT rt)

We need an explicit CMake option like -DCMAKE_FIND_ROOT_PATH=/usr/aarch64-linux-gnu.

For some reasons on some distributions or architectures it is not required to explicitly pass -lrt but on some, it is required to do it explicitly.

We may guess that root path in some ways, when it is not set. For example using <compiler> -dumpmachine.

[slipher]

Using find_library with rt seems weird. Probably we should just be using target_link_libraries(<target> rt). Then CMake will just pass in -lrt and let the linker find it itself.

But I suppose there are plenty of other libs, say SDL, that we might legitimately want to find sometimes.