examples: build some samples with meson

examples: build some samples with meson

Add support for having selected example apps built as part of a meson,
ninja build. By default none are built, and those to be built should be
named directly in the -Dexamples='' meson configuration argument.

This is useful for developers working on a feature who want to use a
suitable example, or examples, to test that feature, as they can compile
everything up in one go, and run the example without having to do a ninja
install first.

This commit adds examples which don't consist of multiple apps in
subdirectories to the meson build, so they can be built by default by
passing -Dexamples parameter to meson.

Not included are the following examples:
* ethtool
* multi-process
* netmap_compat
* performance-thread
* quota_watermark
* server_node_efd
* vm_power_manager

To test the apps added here, use the following command, merged to one line,
to add them to your meson build (command to be run inside the build
directory):

meson configure -Dexamples=bbdev_app,bond,cmdline,distributor,\
	eventdev_pipeline_sw_pmd, exception_path,helloworld,\
	ip_fragmentation,ip_pipeline,ip_reassembly, ipsec-secgw,\
	ipv4_multicast,kni,l2fwd-cat,l2fwd-crypto,l2fwd-jobstats,\
	l2fwd-keepalive,l2fwd,l3fwd-acl,l3fwd-power,l3fwd-vf,l3fwd,\
	link_status_interrupt,load_balancer,packet_ordering,ptpclient,\
	qos_meter,qos_sched,rxtx_callbacks,skeleton,tep_termination,\
	timer,vhost,vhost_scsi,vmdq,vmdq_dcb

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Reviewed-by: Luca Boccassi <bluca@debian.org>