The library quadmath.so is in the gcc-libs package, but the headers quadmath.h and quadmath_weak.h needed to use it are in the gcc-fortran package. Please move the headers into the gcc package so that they can be used from a C program without installing the fortran compiler.

This won't prevent fortran users from using the library, as gcc-fortran depends on gcc, which depends on gcc-libs, so installing fortran will still bring in both the quadmath.so and the quadmath.h files.

The same applies to the multilib version of gcc.