"PPC (G3) machine rather than Intel (which shouldn't make a difference)" 
... but did in this case. Perhaps it's too early in the morning yet but I 
don't see why that error doesn't show up for me when building trunk on an 
Intel machine under 10.6, i.e. this gives no error:
/usr/bin/gcc-4.0 -c -arch ppc -arch i386 -isysroot 
/Developer/SDKs/MacOSX10.4u.sdk  -fno-common -dynamic -DNDEBUG -g -fwrapv 
-O3 -Wall -Wstrict-prototypes  -I. -IInclude -I./Include   -DPy_BUILD_CORE 
-o Python/pymath.o Python/pymath.c
(This is with one patch to configure to fix the test for Darwin 10)