For some reason, the default setting in all Toolchain.cmake
is:
```
set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY BOTH)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE BOTH)
```
For cross-compilation, CMAKE_FIND_ROOT_PATH_MODE_PROGRAM=NEVER
makes sense (we only want to find executables in host system root),
but CMAKE_FIND_ROOT_PATH_MODE_LIBRARY/INCLUDE should be set
to ONLY, otherwise cmake may find libraries in host system root,
and try to link with incompatible libraries (instead of
cross-compilation sysroot).
Fixes#431.
