CMake: include(CTest) only when building tests.

This is needed because ENABLE_TESTING is no longer enabled by
default.

This is needed in order to support usage of this library as a
subdirectory in a parent project.  In that situation, prior
to this change, the inclusion of CTest would unconditionally
enable BUILD_TESTING which would then bleed into other parts
of the project.

Also added some comments explaining how this logic works.

Fix some minor warnings found with a recent clang version

Perhaps this casting should not happen to begin with, but better
to make it explicit where it is happening for readability.  This
fixes a compiler warning.

Fix exceptions on file size calculation on Windows XP x64 and Windows Server 2003 x64

For some reason, `_fstat64` fails with errno 22 on Windows Server 2003 x64 when compiled using the `v141_xp` toolset.
Using `_filelengthi64` instead solves this issue