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/cryptenroll/meson.build

@@ -27,6 +27,7 @@ executables += [
                         libfido2_cflags,
                         libopenssl_cflags,
                         libp11kit_cflags,
+                        tpm2_cflags,
                 ],
         },
 ]