build: use a configuration file for internal features

This avoids re-building the whole project when switching one
Meson option. This shrinks down the compiler invocation command
line, making it more readable and making it easier to inspect
which flags are passed in (the generated file can be opened).
Additionally this is more consistent with our external feature
handling, which uses <wlr/config.h> already.

include/backend/x11.h

@@ -9,10 +9,6 @@
 #include <xcb/xcb.h>
 #include <xcb/present.h>
 
-#if HAS_XCB_ERRORS
-#include <xcb/xcb_errors.h>
-#endif
-
 #include <pixman.h>
 #include <wlr/backend/x11.h>
 #include <wlr/interfaces/wlr_keyboard.h>
@@ -21,6 +17,12 @@
 #include <wlr/types/wlr_pointer.h>
 #include <wlr/render/drm_format_set.h>
 
+#include "config.h"
+
+#if HAS_XCB_ERRORS
+#include <xcb/xcb_errors.h>
+#endif
+
 #define XCB_EVENT_RESPONSE_TYPE_MASK 0x7f
 
 struct wlr_x11_backend;

include/meson.build

@@ -30,3 +30,11 @@ install_subdir('wlr',
 	install_dir: get_option('includedir'),
 	exclude_files: exclude_files,
 )
+
+foreach name, have : internal_features
+	internal_config.set10('HAS_' + name.underscorify().to_upper(), have)
+endforeach
+wlr_files += configure_file(
+	output: 'config.h',
+	configuration: internal_config,
+)

include/xwayland/xwm.h

@@ -5,10 +5,12 @@
 #include <wlr/config.h>
 #include <wlr/xwayland.h>
 #include <xcb/render.h>
+#include "config.h"
+#include "xwayland/selection.h"
+
 #if HAS_XCB_ERRORS
 #include <xcb/xcb_errors.h>
 #endif
-#include "xwayland/selection.h"
 
 /* This is in xcb/xcb_event.h, but pulling xcb-util just for a constant
  * others redefine anyway is meh

meson.build

@@ -99,6 +99,7 @@ internal_features = {
 	'xcb-errors': false,
 	'egl': false,
 }
+internal_config = configuration_data()
 
 wayland_project_options = ['tests=false', 'documentation=false']
 wayland_server = dependency('wayland-server',
@@ -158,13 +159,6 @@ subdir('xwayland')
 
 subdir('include')
 
-foreach name, have : internal_features
-	add_project_arguments(
-		'-DHAS_@0@=@1@'.format(name.underscorify().to_upper(), have.to_int()),
-		language: 'c',
-	)
-endforeach
-
 wlr_inc = include_directories('include')
 proto_inc = include_directories('protocol')