Where are C header files in gcc 7?

On a linux server which has an old version of gcc (4.4.7), I built gcc 7.4.0 from source code and installed it in a directory within the home.

Now I'm not sure how to use it. The directory includes some subdirectories, one named "include". Inside it there is a folder named "c++". It includes another folder named "7.4.0". But where are C header files? Did the installer install them in a different place? Which address should I provide with -I option when compiling a C program?

Unfortunately I don't have the gcc source directory anymore.