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/network/meson.build

@@ -204,9 +204,12 @@ executables += [
                         libsystemd_network,
                         networkd_link_with,
                 ],
+                'dependencies' : [
+                        libbpf_cflags,
+                ],
                 'bpf_programs': [
                         'sysctl-monitor',
-                ]
+                ],
         },
         libexec_template + {
                 'name' : 'systemd-networkd-wait-online',
@@ -225,6 +228,9 @@ executables += [
                         libsystemd_network,
                         networkd_link_with,
                 ],
+                'dependencies' : [
+                        libselinux_cflags,
+                ],
         },
         libexec_template + {
                 'name' : 'systemd-network-generator',