Fix: Conditionally disable tests requiring shared libs support

When building lttng-ust with shared library support explicitly
disabled (e.g.: ./configure --disable-shared), libtool fail with
a fatal error:

  CC       tp.lo
  CC       tp2.lo
  CCLD     liblttng-ust-provider-ust-tests-demo.la
libtool: link: can not build a shared library
libtool: link: See the libtool documentation for more information.
libtool: link: Fatal configuration error.

The build should not fail because some tests require explicit shared
library support, instead they should be skipped.

This patch detect that the --disable-shared flag was passed to the
configure script and toggle the "NO_SHARED" Automake variable.
Thus, the tests that require explicit shared library support can
be skipped when the NO_SHARED variable is true.

[ Edit by Mathieu Desnoyers: add "" in configure.ac to follow the local
  coding style. ]

Signed-off-by: Christian Babeux <christian.babeux@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>