changes to be able to get *working* isolinux result

i spent quite some time to get the isolinux built, as it failed each time with some missing program or header. so i ended up with this patch, in ho it will be helpful in the future to some other users


do not remove tarball until build has finished, as the build process is
fragile and dependencies that are needed are listed nowhere, meaning i
have to download several times as appeared some tool or header was
missing

ignoring and hiding errors is bad and evil, instead stop on errors so
user can see error and try to fix it

additionally fill out requirements in comment. really should test for
program existence or headers to be very nice with it