question about libgcc, libspuc++ in the l4re

Hi,

I find in the src/l4/pkg directory, there are some libraries --
libgcc, libstdc++-v3, and libsupc++. But in those libraries, I just
found very little source code. Instead, they include many makefiles.

Where is the source code of those libraries? And why are there those makefiles?

Thanks a lot.
Yuxin