cmake: require CMake v3.10.2

cmake: require CMake v3.10.2

since we dropped the support of xenial, we now have the luxury of using
newer CMake! and by using CMake 3.10.2, we can prevent libfmt from
assuming that we are using C++11, and hence set `CMAKE_CXX_STANDARD` to
11, which will literally append `-std=gnu++11` to `CMAKE_CXX_FLAGS`.
the last `-std` option passed to `g++` takes precendence.
since we've switched over to C++17, and we are using C++17 features.
so, using cmake older than 3.8 breaks the build. because it is CMake 3.8
which stared support `CMAKE_CXX_STANDARD` 17.

- for bionic: https://packages.ubuntu.com/bionic/cmake : 3.10.2
- for CentOS7:
https://dl.fedoraproject.org/pub/epel/7/x86_64/Packages/c/ : 3.13.5

so in this change,

* bump up the required version to v3.10.2
* cleanups to wipe out the workaround for lower CMake versions
* use `PROJECT_VERSION` defined by `project()` command instead of
  `VERSION` explicitly defined.

Signed-off-by: Kefu Chai <kchai@redhat.com>