NE10 build warning with Cmake 3.19.1

[peter-toft-greve]

Hi with cmake 3.19.1 there is an "early warning" that cmake support for 2.8 and earlier will be deprecated "within soon" :)
I suggest to update https://github.com/projectNe10/Ne10/blob/master/CMakeLists.txt#L27 to 3.15 or alike.

<somepath>/ne10/CMakeLists.txt:27 (cmake_minimum_required):
--
26-Nov-2020 13:33:08 | Compatibility with CMake < 2.8.12 will be removed from a future version of
26-Nov-2020 13:33:08 | CMake.
26-Nov-2020 13:33:08 |  
26-Nov-2020 13:33:08 | Update the VERSION argument <min> value or use a ...<max> suffix to tell
26-Nov-2020 13:33:08 | CMake that the project does not need compatibility with older versions.

I assume that you might be concerned with RHEL7 - and if so then I suggest we update in the same way as
Mbed-TLS/mbedtls#3802

Comments? I can certainly make the PR if you prefer.

[peter-toft-greve]

One suggestion is https://github.com/peter-toft-greve/Ne10/blob/avoid-ne10-warning-with-cmake-3-19/CMakeLists.txt

[peter-toft-greve]

PR opened #274