libteeacl: condition libteeacl with WITH_TEEACL

Build and embed libteeacl upon WITH_TEEACL=1 (default configuration).
This configuration switch allows one to build OP-TEE client without
dependencies on pkg-config and libuuid  when OP-TEE ACL for
PKCS11 is not needed:
 cmake -DWITH_TEEACL=0 ...
or
 make WITH_TEEACL=0 ...

With the comments below addressed, LGTM.

Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Eero Aaltonen <eero.aaltonen@vaisala.com>
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>