linux-flags: remove -Wsuggest-attribute, not supported by gcc 4.6.3
linux/seccomp: fail nicely if siginfo_t->si_syscall is not defined

Ubuntu 12.04 LTS (used by travis-ci.org) provides seccomp.h but does not
define si_syscall in siginfo_t structure in /usr/include/bits/siginfo.h.
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