Add cross-project LTO

[foxtran]

This PR adds TRACY_LTO flag that enables Link-Time optimizations for Tracy. It should improve performance of Tracy calls.

[wolfpld]

I would expect users to set set(CMAKE_INTERPROCEDURAL_OPTIMIZATION ON) in their build scripts.

[foxtran]

Yep, setting CMAKE_INTERPROCEDURAL_OPTIMIZATION is more convenient way, however not all projects can be compiled with LTO (compilation time, linking issues, compiler bugs, and so on), so it would be useful to provide an LTO option for users who can not use LTO on whole project.

[wolfpld]

Yep, setting CMAKE_INTERPROCEDURAL_OPTIMIZATION is more convenient way, however not all projects can be compiled with LTO (compilation time, linking issues, compiler bugs, and so on), so it would be useful to provide an LTO option for users who can not use LTO on whole project.

Please add a note about this in the documentation.