Disallow building static librairies

We don't officialy support static linking the LTTng-UST tracer, however
the autotools build system still allows building static libraries.

Make it impossible to build static libraries without modifying the
configure.ac script to make it explicit.

Adding support for static linking would require a lot of additional
testing an also for applications to explicitly call the library
constructor.

Change-Id: I55d7d9db9fa9c8623305901d085aef1a33286f28
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>