Binaries built on old boxes normally run on newer
ones, so it's not advisable to build binaries
on bleeding edge ones if you want to aim
It may take literally years until enterprise
distributions will have glibc >=2.15. You
seem to have very patient customers ...
You can take your backup of your old linux installation,
put it into some place on your new installation
and do a chroot into this place, and build from there.
You can use the program "apgcc" which is a wrapper
around gcc, it tries to strip incompatible objects.
Not any more available on the net, and does not always
work. I could give you a copy if you are interested.
You can check the generated code with
objdump -T rainlendar2 |grep GLIBC_
The only offending object seems to origin from the
stack-protector, so compiling with
-fnostack-protector might help.