work around an optimizer performance bug in clang.

It seems like some optimization passes of CLang which are enabled by
-O2 (at least in my version, 3.3) do not like nested scopes and long
functions too much. Thus, slightly change the generated source. Timing
on my (quite beefy) machine:

without patch:

make
rm -rf generated-source
time make
[...]
real    10m31.110s
user    10m16.264s
sys     0m13.672s

with patch:

make
rm -rf generated-source
time make
[...]
real    0m47.011s
user    0m44.670s
sys     0m1.968s

the memory used by the compiler goes from 28.8GB to about 330 MB. (I
suppose not everybody has 32 Gigs of memory yet. ;)