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/tpm2-setup/meson.build

@@ -11,6 +11,7 @@ executables += [
                 ],
                 'dependencies' : [
                         libopenssl_cflags,
+                        tpm2_cflags,
                 ],
         },
         libexec_template + {
@@ -21,6 +22,9 @@ executables += [
                         'HAVE_OPENSSL',
                         'HAVE_TPM2',
                 ],
+                'dependencies' : [
+                        tpm2_cflags,
+                ],
         },
         libexec_template + {
                 'name' : 'systemd-tpm2-swtpm',
@@ -39,6 +43,9 @@ executables += [
                         'HAVE_OPENSSL',
                         'HAVE_TPM2',
                 ],
+                'dependencies' : [
+                        tpm2_cflags,
+                ],
         },
 ]