[Nauty] Squeezing out another 10%...

[Gordon Royle]

Hi,

I have discovered through experimentation that I get an additional 10% 
performance out of geng (and hence presumably) nauty by altering the 
optimization flags from the supplied defaults... I am using a Pentium 4 
machine, and if I put

	gcc -O2 -Dmarch=pentium4

then it is about 10% faster than

	gcc -O4

Interestingly, it proves better to use O1/O2 with the 
architecture-specific flag than to use O3 or higher...

Gordon