Comment 0 for bug 1886197

[impact]

systemd FTBFS in b because __NR__sysctl was force-defined by libseccomp in 2.4.1-0ubuntu0.18.04.2, but that force-defining was (correctly) reverted in the latest version 2.4.3-1ubuntu3.18.04.2.

On arm64, __NR__sysctl is not defined, so the build now fails, e.g.:
https://launchpadlibrarian.net/486997294/buildlog_ubuntu-bionic-arm64.systemd_237-3ubuntu10.42~202007031245~ubuntu18.04.1_BUILDING.txt.gz

../src/test/test-seccomp.c: In function ‘test_protect_sysctl’:
../src/test/test-seccomp.c:251:5: error: "__NR__sysctl" is not defined, evaluates to 0 [-Werror=undef]
 #if __NR__sysctl > 0
     ^~~~~~~~~~~~

[test case]

build systemd on bionic with latest packages (i.e. with latest libseccomp2 package)

[regression potential]

this adds a defined() check before comparing the __NR__sysctl value, so any regression would be limited to the specific function test_protect_sysctl() which is part of the test framework, thus leading to a failed build or incorrectly passed/failed test.

[scope]

this is needed only in Bionic.