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/basic/compress.c

@@ -6,25 +6,40 @@
 #include <unistd.h>
 
 #if HAVE_XZ
+#ifndef SYSTEMD_CFLAGS_MARKER_LIBXZ
+#  error "missing libxz_cflags in meson dependency."
+#endif
 #include <lzma.h>
 #endif
 
 #if HAVE_LZ4
+#ifndef SYSTEMD_CFLAGS_MARKER_LIBLZ4
+#  error "missing liblz4_cflags in meson dependency."
+#endif
 #include <lz4.h>
 #include <lz4frame.h>
 #include <lz4hc.h>
 #endif
 
 #if HAVE_ZSTD
+#ifndef SYSTEMD_CFLAGS_MARKER_LIBZSTD
+#  error "missing libzstd_cflags in meson dependency."
+#endif
 #include <zstd.h>
 #include <zstd_errors.h>
 #endif
 
 #if HAVE_ZLIB
+#ifndef SYSTEMD_CFLAGS_MARKER_LIBZ
+#  error "missing libz_cflags in meson dependency."
+#endif
 #include <zlib.h>
 #endif
 
 #if HAVE_BZIP2
+#ifndef SYSTEMD_CFLAGS_MARKER_LIBBZIP2
+#  error "missing libbzip2_cflags in meson dependency."
+#endif
 #include <bzlib.h>
 #endif