cmake: allow use libzstd in system

cmake: allow use libzstd in system

since we are moving the test nodes from bionic to focal, we are able to
use the prebuilt libzstd libraries when running "make check". to speed
up the build and test, in this change:

* add FindZstd.cmake which allows us to use the libzstd in system
* extract BuildZstd.cmake for better readability
* add an option named "WITH_SYSTEM_ZSTD", which defaults to "OFF",
  so user can enable it on demand.

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 16fd07244dd25b46ab1b5a9a3180a354d13c9245)

Conflicts:
	src/compressor/zstd/CMakeLists.txt: minor resolution