replace Julia's gcc dlls with WinRPM versions during deps/build.jl

handle failure somewhat gracefully, provide informative messages

@vtjnash @ihnorton if no objections I'll want to merge and tag this pretty soon, works on my machine for both 32 and 64 bit, and I think I've handled failure cases here
would that just be `catch err`, and add a line `"Error was: $err\n" *` in the warning?