configure: detect cmocka, optional build of unittests

configure.ac

@@ -32,6 +32,7 @@ PKG_PROG_PKG_CONFIG
 # Check for dependencies
 PKG_CHECK_MODULES([libuv], [libuv])
 PKG_CHECK_MODULES([libknot], [libknot])
+PKG_CHECK_MODULES([cmocka], [cmocka], AC_CONFIG_FILES([tests/Makefile]))
 
 # Search other libraries
 AC_SEARCH_LIBS([mdb_env_open], [lmdb])
@@ -40,9 +41,9 @@ AC_SEARCH_LIBS([mdb_env_open], [lmdb])
 AC_CONFIG_FILES([Makefile
 		 lib/Makefile
 		 daemon/Makefile
-		 tests/Makefile
 		 ])
 
+
 # Output
 AC_OUTPUT
 AC_MSG_RESULT([