build: Allow selection of TSS library.

* configure.ac: New option --with-tss to force the use of a
specific TSS library.

While most systems will probably have only one of the two TPM
libraries that we support (the IBM TSS or the Intel TSS), it
would still be helpful to allow which one to use in the event
that both are detected, instead of always using the IBM one.

This patch does that by adding a --with-tss=TSS configure-time
option, where TSS can be "ibm", "intel", or "autodetect". The
default value is "autodetect", which triggers the original
behavior (i.e. try to detect both libraries, and prefer the IBM
one if both are found).

• Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>