Use _FILE_OFFSET_BITS=64 when building on Linux 32-bit systems

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).

Remove unused variable

Minor cmake code duplication improvement

Be more consistent with the existing code and with the naming
_function -> function

Create a macro to add tests for both static/header only.

The only differneces between these two are the libraries they link
with and the target names. Created the simple macro:

_spdlog_prepare_test(<target> <spdlog_lib>)

which does the work.
Signed-off-by: Andrei-Florin BENCSIK <andrei.bencsik@gmail.com>

Fix some spelling errors.

casese -> cases (1)
chache -> cache (1)
cirucal -> circular (1)
dependecy -> dependency (1)
detrmine -> determine (2)
eavluate -> evaluate (1)
exertnal -> external (1)
ony -> only (1)
registation -> registration (3)
registring -> registering (2)
regsistration -> registration (3)
seperate -> separate (2)
wit -> with (1)
withe -> with (1)