build: make gpudev optional

This library can be made optional.
drivers/gpu and app/test-gpudev depend on this library,
so they are automatically disabled if the lib is disabled.

Signed-off-by: Elena Agostini <eagostini@nvidia.com>

doc/guides/prog_guide/gpudev.rst

@@ -23,6 +23,9 @@ Out of scope of this library is to provide a wrapper for GPU specific libraries
 on the device or create GPU specific objects
 (e.g. CUDA Driver context or CUDA Streams in case of NVIDIA GPUs).
 
+This library is optional in DPDK
+and can be disabled with ``-Ddisable_libs=gpudev``.
+
 
 Features
 --------

lib/meson.build

@@ -67,6 +67,7 @@ libraries = [
 
 optional_libs = [
         'bitratestats',
+        'gpudev',
         'gro',
         'gso',
         'kni',