-O0 nearly twice as fast to build as -O2

[Dan Kegel]

OK, this was confusing.  I was getting repeatable five minute
builds, i.e. "make clean; reboot; time make -j3" (more or less :-)
was reliably showing five minutes wall clock time.

Then I did "make distclean; ./configure; make depend".

After that, I got repeatable nine minute builds, i.e.
make clean; reboot; time make -j3
was showing nine minute wall clock time.

Turns out, the difference was... I had been building without
optimization.  So configuring with CFLAGS="-g -O0" is
almost a 2x speedup!

This is with gcc-4.2.3.

Might be handy to know when doing regressions.