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/veritysetup/meson.build

@@ -8,7 +8,10 @@ executables += [
         libexec_template + {
                 'name' : 'systemd-veritysetup',
                 'sources' : files('veritysetup.c'),
-                'dependencies' : libcryptsetup_cflags,
+                'dependencies' : [
+                        libcryptsetup_cflags,
+                        tpm2_cflags,
+                ],
         },
         generator_template + {
                 'name' : 'systemd-veritysetup-generator',