Also, upgrade to CMake 3.13.

The commit also contains significant refactors, as otherwise it is not
possible to properly export target `megengine` to
MegEngine-targets.cmake:

1. Optionally use system provided Flatbuffers.
2. Optionally use system provided MKL-DNN (Tested with Debian).
3. Refactor megbrain and megdnn targets into object libraries.
4. Set different path in BUILD_INTERFACE and INSTALL_INTERFACE of
   various target_include_directories.
5. Specify PUBLIC/PRIVATE on various target_link_libraries.

GitOrigin-RevId: df118a879e12acdc42b56bdd7c7c71b34b0effc3

host build and cross build

now cmake status:
a: host build
    1: windows build -- ok
    2: linux build -- ok
    3: macos build -- ok
b: cross build
    1: windows cross build arm-android -- ok
    2: windows cross build arm-linux   -- ok
    3: linux cross build arm-android -- ok
    4: linux cross build arm-linux   -- ok
    5: macos cross build arm-android -- ok
    6: macos cross build arm-linux   -- ok
    7: macos cross build ios         -- ok
GitOrigin-RevId: f7f376fe8c522410568c304a62310df823c70b65

GitOrigin-RevId: f0c8338beb9cac953bd2d8b76710790940dc9300