Downgrade minimum cmake required to 2.8.12

[nullptr-leo]

In some embedded system, the cmake version is still 2.8.x. So use
SET instead of APPEND to adapt to more envir.

[nullptr-leo]

@nihui Seems CI apt network error?

[nihui]

you would probably have to test cmake version string and add more if - else branches for old cmake versions.

[nihui@nihuini-LC1 build-28]$ /home/nihui/下载/cmake-2.8.0-Linux-i386/bin/cmake ..
-- CMAKE_INSTALL_PREFIX = /home/nihui/osd/ncnn/build-28/install
-- The C compiler identification is GNU
-- The CXX compiler identification is GNU
-- Check for working C compiler: /usr/lib64/ccache/gcc
-- Check for working C compiler: /usr/lib64/ccache/gcc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: /usr/lib64/ccache/c++
-- Check for working CXX compiler: /usr/lib64/ccache/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
CMake Error at src/CMakeLists.txt:234 (target_include_directories):
  Unknown CMake command "target_include_directories".


-- Configuring incomplete, errors occurred!

[nullptr-leo]

you would probably have to test cmake version string and add more if - else branches for old cmake versions.

Actually the version of cmake in my embeded system toolchain is 2.8.12.2. I'll do more tests.

[nihui]

[nihui@nihuini-LC1 build-28]$ /home/nihui/下载/cmake-2.8.12-Linux-i386/bin/cmake ..
-- CMAKE_INSTALL_PREFIX = /home/nihui/osd/ncnn/build-28/install
-- The C compiler identification is GNU 9.2.1
-- The CXX compiler identification is GNU 9.2.1
-- Check for working C compiler: /usr/lib64/ccache/cc
-- Check for working C compiler: /usr/lib64/ccache/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: /usr/lib64/ccache/c++
-- Check for working CXX compiler: /usr/lib64/ccache/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Try OpenMP C flag = [-fopenmp]
-- Performing Test OpenMP_FLAG_DETECTED
-- Performing Test OpenMP_FLAG_DETECTED - Success
-- Try OpenMP CXX flag = [-fopenmp]
-- Performing Test OpenMP_FLAG_DETECTED
-- Performing Test OpenMP_FLAG_DETECTED - Success
-- Found OpenMP: -fopenmp  
-- Looking for include file pthread.h
-- Looking for include file pthread.h - found
-- Looking for pthread_create
-- Looking for pthread_create - not found
-- Looking for pthread_create in pthreads
-- Looking for pthread_create in pthreads - not found
-- Looking for pthread_create in pthread
-- Looking for pthread_create in pthread - found
-- Found Threads: TRUE  
-- Found PROTOBUF: /usr/lib64/libprotobuf.so  
CMake Error at tools/caffe/CMakeLists.txt:11 (target_compile_features):
  Unknown CMake command "target_compile_features".


-- Configuring incomplete, errors occurred!
See also "/home/nihui/osd/ncnn/build-28/CMakeFiles/CMakeOutput.log".
See also "/home/nihui/osd/ncnn/build-28/CMakeFiles/CMakeError.log".