devtools: allow custom set of examples in build test

devtools: allow custom set of examples in build test

To test the installation process of DPDK using "ninja install"
test-meson-builds.sh builds a subset of the examples using "make". To allow
more flexibility for people testing, allow the set of examples chosen for
this make test to be overridden using variable "DPDK_BUILD_TEST_EXAMPLES"
in the environment.

Since a number of example apps link against drivers directly even for
shared builds, we need to ensure that LD_LIBRARY_PATH points to the main
DPDK lib folder so any dependencies of those drivers can be found e.g. that
the PCI/vdev bus driver .so is found. [All drivers are symlinked from
drivers dir back to lib dir on install, so only one dir rather than two is
needed in the path.]

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: David Marchand <david.marchand@redhat.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>