configure: Enforce tss-tss2 if tpm plugin is enabled

The plugin is useless without a TSS 2.0 and this is currently the only one.

configure.ac

@@ -216,7 +216,7 @@ ARG_ENABL_SET([ext-auth],       [enable plugin calling an external authorization
 ARG_ENABL_SET([ipseckey],       [enable IPSECKEY authentication plugin.])
 ARG_ENABL_SET([keychain],       [enables OS X Keychain Services credential set.])
 ARG_ENABL_SET([pkcs11],         [enables the PKCS11 token support plugin.])
-ARG_ENABL_SET([tpm],            [enables the TPM plugin.])
+ARG_ENABL_SET([tpm],            [enables the TPM plugin to access private keys and certificates bound to a TPM 2.0.])
 ARG_DISBL_SET([revocation],     [disable X509 CRL/OCSP revocation check plugin.])
 ARG_ENABL_SET([whitelist],      [enable peer identity whitelisting plugin.])
 ARG_DISBL_SET([xauth-generic],  [disable generic XAuth backend.])
@@ -500,6 +500,10 @@ if test x$aikgen = xtrue; then
 	tss_trousers=true
 fi
 
+if test x$tpm = xtrue; then
+	tss_tss2=true
+fi
+
 if test x$gmp = xtrue -o x$ntru = xtrue -o x$bliss = xtrue; then
 	mgf1=true
 fi