x86 and x86-64 specific stuff:

All this assumes you're using gcc as your compiler.

Add -march=FOO to your CFLAGS, where FOO is your processor: athlon,
athlon-xp, pentium4, core2, etc. (Not all gcc versions support all
these; see the documentation for your installation.)

For example (Requires gcc 4.3 or better):
CFLAGS="-O -march=core2" ./configure -C

On 32 bit systems, consider adding -momit-leaf-frame-pointer to your
CFLAGS. (This might someday be done for you by configure).

On Pentium-M and better, add --enable-sse2 to your configure options
to get a SIMD version of the pseudo random number generator and
some other code.

On Prescott Pentium-4 and better, add --enable-sse3 to your configure
options (Implies --enable-sse2).

Better yet, pass the correct -march=FOO flag in CFLAGS and they'll be
automatically configured.

