Include paths and preprocessor macros for C/C++ indexer and other Language Settings are supplied by Language Settings Providers using Scanner Discovery mechanism.
The entries can be inspected and set up on project properties page "Preprocessor Include Paths, Macros, etc." - Entries tab.
Language Settings Providers can be set up and configured for the selected configuration on project properties page "Preprocessor Include Paths, Macros, etc." - Providers tab.
C/C++ Indexer
Scanner Discovery
Setting up include paths and macros for C/C++ indexer
C/C++ Preferences: Scanner Discovery
C/C++ Project properties: Preprocessor Include Paths, Macros, etc. - Entries tab
C/C++ Project properties: Preprocessor Include Paths, Macros, etc. - Providers tab