Create a tiny fake version of landlock with just the features you're testing for. Since it's only checking for 4 #defines in 3 header files, that's easy. Then

Create a tiny fake version of landlock with just the features you're testing for. Since it's only checking for 4 #defines in 3 header files, that's easy. Then compile your test program against your fake header files (with -Imy-fake-includes). It should compile without errors even if landlock is missing from your actual system. Then build your test program a second time, this time against the real system headers, to test whether landlock is supported on your system.