math/fftw3: Add missing FFTW3LibraryDepends.cmake
This file would normally be generated by cmake, but since we're using
the more feature-complete and stable autotools configure, instead of
pulling in cmake, just provide a pre-generated template file. This
fixes finding fftw3 with future krita.
Install the cmake configuration files in separate directories for each
flavor so that the exported targets are correctly defined.