SPDLOG_BUILD_TESTING now depends on BUILD_TESTING
SPDLOG_BUILD_TESTING now depends on BUILD_TESTING

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