This might not be the best solutions, but at least it works for me.
During the Installation, when you come to a point of selecting type of installation, select "full installation".
After that, scroll down to the middle and you would find 2 categories :
1. CPU Optimization for Scilab
2. FFTW interface for Scilab
These are the 2 "math lib" you mentioned which required internet.
Choose the other option which do not started with "Download", such as the "Altas lib for scilab" for CPU Optimization, and "Reference FFTW lib" for FFTW interface, you should be able to run the scilab withouh any problem.
Some extra notes from my understanding.
1. Indeed if you were to use IPD toolbox for image processing, you should select the atlab lib.
2. FFTW options is for you to use the FFTW lib for performaing various transform, however, the standard fft, ffft2 already included in scilab.
Sorry if I made any mistake, good luck!