xmacro: declare in header files, instantiate instead of include

This works better with meson, where .h files can be declared as being part of an
executable easily, but I couldn’t find a way to declare
e.g. include/atoms.xmacro as a dependency.

related to #4086

include/xcb.h

@@ -49,8 +49,12 @@
                          XCB_EVENT_MASK_FOCUS_CHANGE |                                                \
                          XCB_EVENT_MASK_ENTER_WINDOW)
 
+#include "i3-atoms_rest.xmacro.h"
+#include "i3-atoms_NET_SUPPORTED.xmacro.h"
+
 #define xmacro(atom) extern xcb_atom_t A_##atom;
-#include "atoms.xmacro"
+I3_NET_SUPPORTED_ATOMS_XMACRO
+I3_REST_ATOMS_XMACRO
 #undef xmacro
 
 extern unsigned int xcb_numlock_mask;