Construct paths relative to the Cuberite sources with PROJECT_SOURCE_DIR, instead of wherever the first CMakeLists.txt file happened to be with CMAKE_SOURCE_DIR.
In Android's case, the latter was in a folder called android/ but that's not the root of the source tree, so any file path built off that root was wrong. This caused file-specific warnings exclusions to fail to apply.
* Replaces AppendVPrintf with fmt::sprintf
* fmt::ArgList now used as a type safe alternative to varargs.
* Removed SIZE_T_FMT compatibility macros. fmt::sprintf is fully portable and supports %zu.
* Adds FLOG functions to log with fmt's native formatting style.
* SelfTests: Added a cEvent stress-test.
* cNetwork: Fixed startup event hang.
The original code used to hang with a ~ 1:50000 chance, because on Linux the cEvent was destroyed before its "Set()" returned.
