bump required CMake version to 3.10.2

bump required CMake version to 3.10.2

Of the platforms Graphviz supports, the one with the oldest CMake version
available is Ubuntu 18.04, which has CMake 3.10.2. This change raises the
baseline CMake requirement to indicate we no longer support CMake 3.9.

The diff between CMake 3.9 and 3.10 does not actually contain anything useful to
Graphviz. But it still seems wise to proactively move our floor forwards.