The build system assumed sizeof (void *) == 8 as a host system
running on a x86_64 architecture, x86 otherwise. However, this
assumption is invalid for most other architectures out there, such as
aarch64 or armv7l.
This PR refers to the CMAKE_HOST_SYSTEM_PROCESSOR variable
which, according to the documentation, typically refers to the output
from "uname -m" for Unix-like systems.
