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/elf-util.c

@@ -1,6 +1,10 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
 #if HAVE_ELFUTILS
+#ifndef SYSTEMD_CFLAGS_MARKER_LIBELF
+#  error "missing libelf_cflags in meson dependency."
+#endif
+
 #include <dwarf.h>
 #include <elfutils/libdwelf.h>
 #include <elfutils/libdwfl.h>