When cross-compiling spdlog from x86-64 to armhf, I ran spdlog-utests
via qemu-arm and noticed that the "daily_logger rotate" test was failing
because count_files always returned zero.

Investigation of count_files revealed that readdir was returning nullptr
immediately and setting errno to 75, i.e. "value too large for defined
data type".  I suspected this had something to do with some 64 vs.
32-bit thing, so I added _FILE_OFFSET_BITS=64 to the build and that
seems to have made readdir happy.

It might be safe to add _FILE_OFFSET_BITS=64 for all Linux builds, but
it only seems to be necessary for the 32-bit case (which is a pretty
small audience these days -- I'm only building for armhf to target a
Raspberry Pi 3 running Raspbian, which runs in 32-bit mode).