Set C++ standard so that it propagates to users

[Bidski]

See #456

Setting the C++ standard in this way causes it to show up in pagmo_export.cmake as an INTERFACE_COMPILE_FEATURE which, in turn, will instruct anyone linking against the pagmo cmake target to compile using the C++17 standard (at minimum). See cmake docs

[bluescarni]

@Bidski that's great, thanks a lot for digging this out.

Could I ask you to make the following further additions to the PR:

• apply the same modification to the CMakeLists.txt of tests, benchmarks and tutorials,
• bump up the minimum required CMake version to 3.8 in the main CMakeLists.txt,
• update the documentation in doc/sphinx/install.rst to reflect the fact that the minimum required CMake version is now 3.8.

If you don't have time/bandwidth for this, I'll do this myself, just let me know.

[Bidski]

@bluescarni I have applied the changes you requested

[bluescarni]

@Bidski great, thanks a lot! I'll merge as soon as the CI runs.