Fix HOST determination in CMake script

Fix HOST determination in CMake script

As per the configure script, HOST variable value should identify the
target, not build host.

* CMakeLists.txt (HOST): Set variable before find_package(Threads);
change message type to STATUS; report ${HOST} value as "TARGET".
* CMakeLists.txt (_HOST): Use CMAKE_SYSTEM_PROCESSOR instead
of CMAKE_HOST_SYSTEM_PROCESSOR; specify vendor as "unknown" (instead
of empty string); remove FIXME.