Here it is... The "Compiler's installation directory" was always empty, but Code Completion worked nevertheless.
Edit: Okay so I was just playing around with the "Compiler's installation directory"... "Auto-detect" always said 'Auto-detected installation path of "GNU GCC Compiler" in "" ', resulting in an empty path as shown below. I first gave it /bin (symlink to /usr/bin on my system), didn't do anything + auto-detect still gave an empty path. Tried /usr/bin, with absolutely no hope, still nothing as expected. Randomly clicked Auto-detect again, it said 'installation path in "/usr" ', and now it works. The fuck did I just experience? It worked before even when the path was empty...