External dependencies (dependencies with a «Usage» stereotype and the CG::Dependency::UsageType property set to Specification/Implementation) and implicit dependencies (such as associations and generalizations) are generated as forward declarations and #include statements.
To generate a local #include statement (for example, #include <C.h>), set the CG::File::IncludeScheme property to LocalOnly.
To generate a relative #include statement (for example, #include <MySubsystem\C.h>), set the CG::File::IncludeScheme property to RelativeToConfiguration.
You can also use the <lang>_CG::Package::GenerateDirectory and CG::Class/Package::FileName properties to set relative paths. See the definition of these properties in the Features window.