tree-wide: check if necessary cflags dependencies are set

This makes each `foo_cflags` dependency define a `SYSTEMD_CFLAGS_MARKER_FOO`
macro, and checks if the macro is set when headers provided by external
libraries are included.

With this, we can fail fast at compile time if necessary `_cflags`
dependencies are omitted in meson.build. Missing dependencies found by this
mechanism have been added across the tree.

src/shared/seccomp-util.h

@@ -7,6 +7,10 @@
 #include "shared-forward.h"
 
 #if HAVE_SECCOMP
+#ifndef SYSTEMD_CFLAGS_MARKER_LIBSECCOMP
+#  error "missing libseccomp_cflags in meson dependency."
+#endif
+
 #include <seccomp.h> /* IWYU pragma: export */
 
 #include "dlfcn-util.h"