* support CMake user registry package registration
* use GNUInstallDirs to set installation firectories
* use spdlog namespace in both build and install interfaces
I encountered an issue when using spdlog through add_subdirectory.
Since SPDLOG_BUILD_TESTING is ON by default, it now adds tests to my project, even if BUILD_TESTING (the official CTest variable) is set to OFF.
cmake_dependent_option makes it so that if someone enables BUILD_TESTING then SPDLOG_BUILD_TESTING will be set to ON by default.
This way one can disable all external tests by setting BUILD_TESTING before using add_subdirectory and then setting it back to its original value.
The only change for those using the library directly is that they now use BUILD_TESTING instead of SPDLOG_BUILD_TESTING when configuring.
This is helpful when using spdlog as a dependency (git submodule) when a
master project is not interested in spdlog tests. Using
"BUILD_TESTING" name may create a confusion.
Extra: BUILD_EXAMPLE variable already have a prefix.
This is a usual CMake way of ensuring that IDEs have a way of showing
all source files which comprise this header-only library. It works in
the Qt Creator, for example.
