Optionally use external spdlog library

[swltr]

As mentioned in #7, it would be nice to allow external dependencies to be used.
This helps e.g. if you already have spdlog as a dependency in your CMake build.
With this small change, it is possible to turn off auto-detecting spdlog.
Leaving the target_link_libraries(spdlog_setup ...) calls as they are, these then simply use an existing spdlog library.
You need to add that library somewhere else in your build, then.
In my case, I have a line like

add_subdirectory(libs/cmake/spdlog-1.3.1)

in my top-level CMakeLists.txt.

[codecov]

Codecov Report

Merging #72 (7d0a773) into master (91ecd72) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #72   +/-   ##
=======================================
  Coverage   88.16%   88.16%           
=======================================
  Files           4        4           
  Lines         566      566           
=======================================
  Hits          499      499           
  Misses         67       67           

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[swltr]

@guangie88 What do you think about this?

[swltr]

@guangie88 Any thoughts on this?