build: Simplify detecting a TPM emulator.

* configure.ac (TPMSERVER): Don't supply hard-coded path.
(SWTPM, SWTPM_IOCTL, TSSSTARTUP): Likewise.

Having hard-coded path has bad side effect; It may not be detected
even if it's available with PATH.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>