/usr/include/net-snmp/system/ is in libsnmp-dev 5.7.3+dfsg-1.7+deb9u1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/net-snmp/system/aix.h
/usr/include/net-snmp/system/bsd.h
/usr/include/net-snmp/system/bsdi.h
/usr/include/net-snmp/system/bsdi3.h
/usr/include/net-snmp/system/bsdi4.h
/usr/include/net-snmp/system/cygwin.h
/usr/include/net-snmp/system/darwin.h
/usr/include/net-snmp/system/darwin10.h
/usr/include/net-snmp/system/darwin7.h
/usr/include/net-snmp/system/darwin8.h
/usr/include/net-snmp/system/darwin9.h
/usr/include/net-snmp/system/dragonfly.h
/usr/include/net-snmp/system/dynix.h
/usr/include/net-snmp/system/freebsd.h
/usr/include/net-snmp/system/freebsd10.h
/usr/include/net-snmp/system/freebsd11.h
/usr/include/net-snmp/system/freebsd12.h
/usr/include/net-snmp/system/freebsd2.h
/usr/include/net-snmp/system/freebsd3.h
/usr/include/net-snmp/system/freebsd4.h
/usr/include/net-snmp/system/freebsd5.h
/usr/include/net-snmp/system/freebsd6.h
/usr/include/net-snmp/system/freebsd7.h
/usr/include/net-snmp/system/freebsd8.h
/usr/include/net-snmp/system/freebsd9.h
/usr/include/net-snmp/system/generic.h
/usr/include/net-snmp/system/hpux.h
/usr/include/net-snmp/system/irix.h
/usr/include/net-snmp/system/kfreebsd.h
/usr/include/net-snmp/system/linux.h
/usr/include/net-snmp/system/mingw32.h
/usr/include/net-snmp/system/mips.h
/usr/include/net-snmp/system/netbsd.h
/usr/include/net-snmp/system/openbsd.h
/usr/include/net-snmp/system/openbsd4.h
/usr/include/net-snmp/system/openbsd5.h
/usr/include/net-snmp/system/osf5.h
/usr/include/net-snmp/system/solaris.h
/usr/include/net-snmp/system/solaris2.3.h
/usr/include/net-snmp/system/solaris2.4.h
/usr/include/net-snmp/system/solaris2.5.h
/usr/include/net-snmp/system/solaris2.6.h
/usr/include/net-snmp/system/sunos.h
/usr/include/net-snmp/system/svr5.h
/usr/include/net-snmp/system/sysv.h
/usr/include/net-snmp/system/ultrix4.h