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/fuzz/meson.build

@@ -8,7 +8,6 @@ simple_fuzzers += files(
         'fuzz-env-file.c',
         'fuzz-hostname-setup.c',
         'fuzz-json.c',
-        'fuzz-pe-binary.c',
         'fuzz-time-util.c',
         'fuzz-udev-database.c',
         'fuzz-user-record.c',
@@ -16,6 +15,13 @@ simple_fuzzers += files(
         'fuzz-varlink-idl.c',
 )
 
+executables += [
+        fuzz_template + {
+                'sources' : files('fuzz-pe-binary.c'),
+                'dependencies' : libopenssl_cflags,
+        },
+]
+
 # The following fuzzers do not work on oss-fuzz. See #11018.
 if not want_ossfuzz
         simple_fuzzers += files('fuzz-compress.c')