Remove or fix relative includes

[Flamefire]

On some systems relative includes are not automatically possible, i.e. the path to the current source file may not automatically added to the include-path.

Currently Boost.Atomic fails on Cygwin 3.1.7 with:

libs\atomic\src\find_address_sse2.cpp:25:10: fatal error: find_address.hpp: No such file or directory
   25 | #include "find_address.hpp"
      |          ^~~~~~~~~~~~~~~~~~
compilation terminated.
libs\atomic\src\find_address_sse41.cpp:25:10: fatal error: find_address.hpp: No such file or directory
   25 | #include "find_address.hpp"
      |          ^~~~~~~~~~~~~~~~~~
compilation terminated.

According to https://www.appveyor.com/docs/windows-images-software/#mingw-msys-cygwin the Appveyor images 2015&2017 use Cygwin 3.0.7 while the newer 2019&2022 use 3.1.7 where those failures appear. Hence I'd suggest to add CI for both.

[Lastique]

Fixed, thanks.