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)

Fix build failure on clang 7 with libc++

Unlike the GNU C++ STL, there's no implicit include for <array> in this
one, apparently.