I haven't looked at sdcc for a while but doesn't it defferenciate between a signal and an interrupt where an interrupt occurs once and is then disabled and a signal is never disabled?
I may be talking giberish here, but why change the habit of a lifetime is what I say.
I haven't looked too closely at your code but that my first thought anyway,if it works in one compiler and not another its usualy some difference in the way the compiler interprets something.