build: fix out-of-tree build

Signed-off-by: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
diff --git a/Makefile b/Makefile
index 71efca4..297f8e7 100644
--- a/Makefile
+++ b/Makefile
@@ -176,7 +176,7 @@
 	if git describe >/dev/null 2>&1; then \
 		git describe > "$@"; \
 	else \
-		cp VERSION "$@"; \
+		cp $(top_srcdir)/VERSION "$@"; \
 	fi
 
 $(INSTALL_DIR)/Version: Version