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/ssl-util.h

@@ -8,6 +8,10 @@
 int dlopen_libssl(int log_level);
 
 #if HAVE_OPENSSL
+#ifndef SYSTEMD_CFLAGS_MARKER_LIBOPENSSL
+#  error "missing libopenssl_cflags in meson dependency."
+#endif
+
 #define LIBSSL_NOTE(priority)                                           \
         SD_ELF_NOTE_DLOPEN("libssl",                                    \
                            "Support for TLS",                           \