build: Make TPM2 support conditional

* configure.ac (HAVE_LIBTSS): New acdefine and am_conditional.
* agent/Makefile.am: (gpg_agent_SOURCES): Move tpm files to ...
(gpg_agent_SOURCES) [HAVE_LIBTSS]: ... here.
* agent/agent.h (divert_tpm2_pksign, divert_tpm2_pkdecrypt)
(divert_tpm2_writekey) [!HAVE_LIBTSS]: Add stub functions.

This adds a configure stanza to check for the necessary libtss to
support TPM functions. If found, the library functions will be
dynamically loaded, meaning that a system built with TPM2 support will
still execute correctly (obviously minus TPM2 support) if installed
without libtss being present.

• Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>