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/microhttpd-util.h

@@ -8,6 +8,10 @@
 int dlopen_microhttpd(int log_level);
 
 #if HAVE_MICROHTTPD
+#ifndef SYSTEMD_CFLAGS_MARKER_LIBMICROHTTPD
+#  error "missing libmicrohttpd_cflags in meson dependency."
+#endif
+
 #define MICROHTTPD_NOTE(priority)                                       \
         SD_ELF_NOTE_DLOPEN("microhttpd",                                \
                            "Support for embedded HTTP server via libmicrohttpd", \