gtk-doc: Fallback to 'cp' when 'cp -u' fails

'cp -u' is not specified in POSIX and it can fail with illegal option
error on systems not supporting it. It is better to fallback to the
slower 'cp' than causing build failure.

https://bugzilla.gnome.org/show_bug.cgi?id=791784

gtk-doc-plugins.mak

@@ -121,7 +121,9 @@ scanobj-build.stamp: $(SCANOBJ_DEPS) $(basefiles)
 	@if test x"$(srcdir)" != x. ; then				\
 	    for f in $(SCANOBJ_FILES) $(SCAN_FILES);			\
 	    do								\
-	        if test -e $(srcdir)/$$f; then cp -u $(srcdir)/$$f . ; fi;	\
+	        if test -e $(srcdir)/$$f; then				\
+	            cp -u $(srcdir)/$$f . || cp $(srcdir)/$$f . ;	\
+	        fi;							\
 	    done;							\
 	fi;								\
 	mkdir -p $(INSPECT_DIR); \
@@ -155,7 +157,9 @@ scan-build.stamp: $(HFILE_GLOB) $(EXTRA_HFILES) $(basefiles) scanobj-build.stamp
 	@if test x"$(srcdir)" != x. ; then				\
 	    for f in $(SCANOBJ_FILES) $(SCAN_FILES);			\
 	    do								\
-	        if test -e $(srcdir)/$$f; then cp -u $(srcdir)/$$f . ; fi;	\
+	        if test -e $(srcdir)/$$f; then				\
+	            cp -u $(srcdir)/$$f . || cp $(srcdir)/$$f .;	\
+	        fi;							\
 	    done;							\
 	fi
 	@_source_dir='' ;						\