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We need at list version 2.4.0 of libseccomp for seccomp-filter.c to
build cleanly.

* configure.ac: Use pkg-config to check for libseccomp.
* lib-src/Makefile.in (HAVE_LIBSECCOMP, LIBSECCOMP_LIBS)
(LIBSECCOMP_CFLAGS): New variables.
(SECCOMP_FILTER, seccomp-filter$(EXEEXT)): Use them.