It breaks build on Alpine/postmarketOS, and is explicitely an error on
more recent version of cpu_features:
https://github.com/google/cpu_features/blob/master/include/cpuinfo_x86.h#L207.

protecting the include with #ifdef fix the problem, as every use of
cpu_features is already protected.

dbus-cpp was dropped from 18.04 as it is not maintained anymore so we
should migrate to something more stable with a long term perspective.
libsystemd fits in nicely here as it has a nice and clean dbus
implementation which is battle tested and can be easily integrated into
our existing C++ code base.

This looks better than leave cmake *.in everywhere in the tree.
Also fix typo according to review comments of PR 425.

In CMakeLists.txt, it already defines:

if (ANBOX_VERSION_SUFFIX)
    set(ANBOX_VERSION "${ANBOX_VERSION}-${ANBOX_VERSION_SUFFIX}")
endif()

So there's no need to add suffix again in code
Signed-off-by: NShengjing Zhu <i@zhsj.me>