[Help needed] Support using pre-compiled ncnn library

[csukuangfj]

Currently, we use a fork of ncnn

sherpa-ncnn/cmake/ncnn.cmake

Line 9 in fc4d3bc

set(ncnn_URL "https://github.com/csukuangfj/ncnn/archive/refs/tags/sherpa-1.1.tar.gz")

But actually, sherpa-ncnn supports using the master of
https://github.com/tencent/ncnn/

We need to change

sherpa-ncnn/CMakeLists.txt

Line 140 in fc4d3bc

include(ncnn)

to something like below

find_package(ncnn)

if(NOT ncnn_FOUND)
  include(ncnn)
endif()

We also need to add tests to cover the above change.

---

Please see
https://github.com/Tencent/ncnn/wiki/use-ncnn-with-own-project
for further information.