Doc: add CMake example

Doc: add CMake example

In doc/examples/cmake-multiple-shared-libraries/, a new example lives.
This example requires a C++ compiler (HAVE_CXX) and it requires also a
cmake executable (HAVE_CMAKE). This example relies on the cmake module
called FindLTTngUST, which was kindly provided by Philippe Proulx.

The alignment of a small amount of lines was improved in the FindLTTngUST
module.

This new example generates a shared library (tracepoint provider) that
links with dl and with lttng-ust. Two other shared libraries are also
generated, and these two are linked with the tracepoint provider shared
library.

Link: https://lists.lttng.org/pipermail/lttng-dev/2016-June/026213.html
Link: https://lists.lttng.org/pipermail/lttng-dev/2016-June/026203.html
Link: https://www.pastery.net/fvanfh/raw/
Acked-by: Philippe Proulx <eeppeliteloop@gmail.com>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Sebastien Boisvert <sboisvert@gydle.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>