Compiler/CCSTUDIO: CCS 9.2 How to define include path variables so I don't have to add them to each project separately ?

Depending on the Products enabled for your project (like DSPLIB, MATHLIB, etc), there will be a set of variables predefined by CCS. If you go to Project Properties->Build->Variables tab and check the "Show system variables" box, you will be able to see the list of variables. Note that there may be variables like ${COM_TI_MAS_MATHLIB_C66X_INCLUDE_PATH} defined but are not set to any value. Many or all of the paths shown in your screenshot may not be resolving to anything. You can confirm by hovering over the three dots and see if it displays a directory or says <none>. If it says <none> then it is not really doing anything.

Regarding adding include_paths in a common file that can be used across multiple projects there isn't a straightforward way to do this. But this thread discusses some options that may be helpful. https://e2e.ti.com/support/tools/ccs/f/81/t/654985

Hope this helps.