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/libsystemd/sd-bus/test-bus-marshal.c

@@ -5,12 +5,18 @@
 #include "macro.h"
 
 #if HAVE_GLIB
+#ifndef SYSTEMD_CFLAGS_MARKER_LIBGLIB
+#  error "missing libglib_cflags in meson dependency."
+#endif
 DISABLE_WARNING_FORMAT_NONLITERAL
 #include <gio/gio.h> /* NOLINT */
 REENABLE_WARNING
 #endif
 
 #if HAVE_DBUS
+#ifndef SYSTEMD_CFLAGS_MARKER_LIBDBUS
+#  error "missing libdbus_cflags in meson dependency."
+#endif
 #include <dbus/dbus.h>
 #endif