This line:
"#{RbConfig::CONFIG['CFLAGS']} #{$CFLAGS}".split(" ").uniq.join(" ").gsub("$(cflags)", "").gsub("ppc", "")

results in the following line for Snow Leopard:
gcc -arch i386 x86_64 -g -Os -pipe -fno-common -DENABLE_DTRACE

This is not valid, because it has to be -arch i386 -arch x86_64 in order to compile it for both architectures. With these flags, the compilations fails with the following error:

i686-apple-darwin10-gcc-4.2.1: x86_64: No such file or directory