Improve version checking

[TheChymera]

the current version checking method doesn't work, and the build system ends up using the deprecated file_string method.

[coalsont]

This appears to depend on cmake/boost version:

https://cmake.org/cmake/help/latest/policy/CMP0093.html#policy:CMP0093

The CMAKE_POLICY(VERSION 2.8.7) line was intended for this sort of thing, but a closer reading explains why it didn't work for the Doxygen warning either: it can only set the minimum version (until 3.12), and therefore never set things to older policy, so it can't be used to make things work in older cmake. Considering these tests are for fairly old versions of boost, breaking older cmake doesn't seem like the right solution here. I think adding another IF (POLICY ...) section may be the easy fix. However, I wonder if boost's own shipped cmake file checks this policy, because if not, something more drastic may be required.