Avoid multiply defined symbols linker errors

[seanharmer]

When using spdlog_setup with a compiled spdlog library.
Fixes #74.

[codecov]

Codecov Report

Merging #75 (0488b2b) into master (d1b3f41) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #75   +/-   ##
=======================================
  Coverage   88.16%   88.16%           
=======================================
  Files           4        4           
  Lines         566      566           
=======================================
  Hits          499      499           
  Misses         67       67           

Impacted Files	Coverage Δ
include/spdlog_setup/details/conf_impl.h	90.15% <ø> (ø)

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