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/bootctl/meson.build

@@ -24,7 +24,10 @@ executables += [
                 ],
                 'sources' : bootctl_sources,
                 'link_with' : boot_link_with,
-                'dependencies' : [libopenssl_cflags],
+                'dependencies' : [
+                        libopenssl_cflags,
+                        tpm2_cflags,
+                ],
         },
         test_template + {
                 'sources' : files(