Renamed TRACY_NO_SYS_TRACE -> TRACY_NO_SYSTEM_TRACING to match the
build flag name. Unlike the meson logic, the CMake logic directly
maps the option name to the build flag that is injected. With the
mismatched name, the flag wasn't being properly applied.
Added TRACY_TIMER_FALLBACK option to expose the same-named flag.
Moved signal.h include to get sigaction definition that was missing when
TRACY_NO_CALLSTACK was defined.
