configure.ac: Allow for the unit tests to be enabled selectively.

This makes the dependency on libcmocka optional.

Signed-off-by: Philip Tricca <flihp@twobit.us>
diff --git a/ChangeLog b/ChangeLog
index 502b7cf..21ed205 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,6 @@
 Unreleased
 Added
+* Allow for unit tests to be enabled selectively.
 * added pkg-config files for libraries
 Changed
 * rename libtss2 to libsapi