gmake keeps going when it shouldn't?

[Stephen O'Loughlin]

Say I have a makefile with the following...

file4.txt: file3.txt
        xterm -e "sleep 4 ; touch file4.txt"

file1.txt:
        xterm -e "sleep 4 ; touch file1.txt"

file2.txt: file1.txt
        xterm -e "sleep 4 ; touch file2.txt"

file3.txt: file2.txt
        xterm -e "sleep 4 ; touch file3.txt"


This makefile creates four files file[1-4].txt by launching an xterm,
waiting for 4 seconds, and then touching the appropriate file.  Now if
I close one of the xterms during the waiting period, the touch command
doesn't get executed and the file isn't created, so you'd expect that
make would stop.  However it doesn't stop and keeps building the
remaining targets.  So if for example I killed the xterm that was
supposed to build file2.txt.  I would end up with file1.txt, file3.txt
and file4.txt, which isn't what I would expect or what I want.

Any ideas?

Thanks.