Fix: Build examples when rpath is stripped from in-build-tree libs

Fix: Build examples when rpath is stripped from in-build-tree libs

Observed issue
==============

Certain tool chains[1, 2] emit warnings or errors when building the
example applications.

```
make[3]: Entering directory
'/home/xxx/src/efficios/lttng/master/src/lttng-ust/doc/examples/easy-ust'
CC       sample.o
CC       tp.o
CCLD     sample

/usr/bin/ld: warning: liblttng-ust-common.so.1, needed by ../../../src/lib/lttng-ust/.libs/liblttng-ust.so, not found (try using -rpath or -rpath-link)
/usr/bin/ld: warning: liblttng-ust-tracepoint.so.1, needed by ../../../src/lib/lttng-ust/.libs/liblttng-ust.so, not found (try using -rpath or -rpath-link)
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_add_fd_to_tracker'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_urcu_after_fork_child'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_common_ctor'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_tp_init'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_tp_probe_register_queue_release'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_cancelstate_disable_pop'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_urcu_synchronize_rcu'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_urcu_register_thread'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_tp_probe_prune_release_queue'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_cancelstate_disable_push'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_urcu_before_fork'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_lock_fd_tracker'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_trace_clock'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_urcu_has_sys_membarrier'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_unlock_fd_tracker'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_tp_exit'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_get_cpu_sym'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_common_alloc_tls'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_urcu_after_fork_parent'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_delete_fd_from_tracker'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_urcu_register'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_urcu_reader'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_urcu_gp'
/usr/bin/ld: ../../../src/lib/lttng-ust/.libs/liblttng-ust.so: undefined reference to `lttng_ust_tp_probe_unregister_queue_release'
collect2: error: ld returned 1 exit status
```

=== Reproducer ===

The easiest way to consistently reproduce this type of build failure
is to perform the following steps:

```
./configure
make -j$(nproc)
find . -iname '*.so' -exec chrpath -d {} \;
make -C doc/examples clean
make -j$(nproc)
```

As the examples are not built with libtool, finding the libraries to
link against depends on the shared objects having an rpath.

E.g.

```
$ chrpath src/lib/lttng-ust/.libs/liblttng-ust.so.1.0.0
src/lib/lttng-ust/.libs/liblttng-ust.so.1.0.0: RUNPATH=/home/xxx/src/efficios/lttng/master/src/lttng-ust/src/lib/lttng-ust-common/.libs:/home/xxx/src/efficios/lttng/master/src/lttng-ust/src/lib/lttng-ust-tracepoint/.libs:/home/xxx/src/efficios/lttng/master/usr/lib
```

The current examples build with `-Wl,-rpath`  for `liblttng-ust`, but
not the dependencies of `liblttng-ust` (which would normally be found
via it's own rpath). If the `rpath` is stripped from
`liblttng-ust.so`, or if the tool chain ignores `rpath` explicitly,
then the build with fail.

In the case of a yocto build environment as in GitHub#61[1], the
following commands reproduced the warnings and the errors seen in the
above test case.

```
git clone git://git.yoctoproject.org/poky && cd poky/
. oe-init-build-env
echo "PACKAGECONFIG:pn-lttng-ust = 'examples'" >>conf/local.conf
bitbake lttng-ust
```

=== Solution ===

Explicitly add library search paths and set linker rpath-link are set
for both the standard and cmake examples. Similar changes were
proposed for each of those parts respectively in GitHub#61[1] and GitHub#63[2].

=== Known issues ===

While the `rpath-link` for the second order library dependencies will
allow the builds to complete, the examples will not work at
runtime even when using `rpath`. From `man ld.so`, the rpaths in an
executable or shared object are only considered for direct
dependencies. Therefore, without setting `LD_LIBRARY_PATH` or
installing the libraries test applications will fail as follows:

```
$ ./doc/examples/easy-ust/sample
./doc/examples/easy-ust/sample: error while loading shared libraries: liblttng-ust-common.so.1: cannot open shared object file: No such file or directory
```

References
==========

[1]: https://github.com/lttng/lttng-ust/pull/61
[2]: https://github.com/lttng/lttng-ust/pull/63

Change-Id: I273ccddd0d0b7a1b57b9e09ddf48d8b5b41e6f8e
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>