back/wayland: build the X11 pasteboard owner into gpbs (#161)

gpbs selects XPbOwner as its pasteboard owner on every platform except
Windows, but xpbs.m, which defines that class, was only compiled for the
x11 server.  A wayland build therefore produced a gpbs with no pasteboard
owner, so NSPasteboard was disconnected from the desktop clipboard and
paste was disabled in GNUstep applications.

Compile xpbs.m and xdnd.c into gpbs for a wayland build when X11 is
available, so the X11 owner reaches the desktop clipboard through XWayland.
Guarding on X11 keeps a wayland build without X11 unchanged.

Author: DTW-Thalion

Tools/GNUmakefile

@@ -54,6 +54,18 @@ gpbs_C_FILES += xdnd.c
 endif
 endif
 
+# The wayland backend has no pasteboard owner of its own, so build the X11
+# owner into gpbs when X11 is available; it reaches the desktop clipboard
+# through XWayland.  Without X11 gpbs stays an internal-only pasteboard.
+ifeq ($(BUILD_SERVER),wayland)
+ifneq ($(findstring -lX11,$(GRAPHIC_LIBS)),)
+gpbs_OBJC_FILES += xpbs.m
+ifeq ($(BACKEND_BUNDLE),yes)
+gpbs_C_FILES += xdnd.c
+endif
+endif
+endif
+
 ifeq ($(BUILD_SERVER),win32)
 gpbs_OBJC_FILES += win32pbs.m 
 endif