cmake: improve CMakeLists.txt
cmake: improve CMakeLists.txt

* support CMake user registry package registration
* use GNUInstallDirs to set installation firectories
* use spdlog namespace in both build and install interfaces

COMBINE-lab/spdlog,godbyk/spdlog,hunter-packages/spdlog,godbyk/spdlog,COMBINE-lab/spdlog,COMBINE-lab/spdlog,godbyk/spdlog,hunter-packages/spdlog,hunter-packages/spdlog