The usual. checking the GLIBC_VERSION version... unsupported version 2.17 configure: error: Valgrind requires glibc version 2.2 - 2.16
Okay, the fix was added to valgrind-3.8.1.ebuild without rev bump. Reopen if this is still an issue. I don't have a glibc-2.17 system, so I can't test, but I don't see how I could mess up here (famous last words :)
same for 2.18 please :)