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/report/meson.build

@@ -10,6 +10,9 @@ executables += [
                         'report-sign.c',
                         'report-upload.c',
                 ),
+                'dependencies' : [
+                        libcurl_cflags,
+                ],
         },
 
         libexec_template + {