How to add C-Libraries in macOS Big Sur?

Question

I want to add a C-Library in order to include it in Code simply by typing:

#include <libname>

It works when I'm using the full path like so:

#include "path/to/lib.h"

Is there a way to add the lib to some kind of standard location, where gcc will find it?

I've tried the following but it didn't work for me: How to add default Include and Library path for gcc in Macos for bash?

(both echo $LD_LIBRARY_PATH and echo $DYLD_LIBRARY_PATH are empty and copying all *.h-Files to /usr/local/ also didn't work.)

Thank you very much in advance for your help!