gpu_sysman: change configure to use pkg-config for dependency

gpu_sysman: change configure to use pkg-config for dependency

And enable Sysman plugin automatically when dependency is found.

While GPU plugin uses just Sysman API subset of the Level-Zero API
family, name of the used library / loader is level-zero => dropped
"--with-sysman" option and renamed configure variables.

Also, now that Level-Zero packages are in the distributions, Sysman
API GPU plugin could be enabled by default when pkg-config finds
level-zero. This simplifies config code.

Signed-off-by: Eero Tamminen <eero.t.tamminen@intel.com>