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/tmpfiles/meson.build

@@ -12,7 +12,10 @@ executables += [
                 'public' : true,
                 'extract' : files('tmpfiles.c') +
                             offline_passwd_c,
-                'dependencies' : libacl_cflags,
+                'dependencies' : [
+                        libacl_cflags,
+                        libselinux_cflags,
+                ],
         },
         executable_template + {
                 'name' : 'systemd-tmpfiles.standalone',
@@ -24,7 +27,10 @@ executables += [
                         libshared_static,
                         libsystemd_static,
                 ],
-                'dependencies' : libacl_cflags,
+                'dependencies' : [
+                        libacl_cflags,
+                        libselinux_cflags,
+                ],
         },
         test_template + {
                 'sources' : files('test-offline-passwd.c') +