Glibc versions prior to 2.5 do not define PTRACE_GETSIGINFO
(determined by Patrick J. LoPresti).
Tweak the ifdeffery.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14565 a5019735-40e9-0310-863c-91ae7b9d1cf9