Introduce a configure-time check for C++17 filesystem

Various compilers have various fun ways of failing to compile when it
is used so let's check if they will work properly during configure
rather than much later.

Change-Id: Ia93d4b91b3d269b4cab2a5f677c3c89e06b44ce3
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>