[valadoc] driver/0.24.x: Don't overwrite VALAFLAGS

[Rico Tzschichholz <ricotz src gnome org>]

commit 1e5a07375bf86d143ed6d833d82e897538b1b672
Author: Rico Tzschichholz <ricotz ubuntu com>
Date:   Sat Jan 18 09:46:41 2014 +0100

    driver/0.24.x: Don't overwrite VALAFLAGS

 src/driver/0.24.x/Makefile.am |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/driver/0.24.x/Makefile.am b/src/driver/0.24.x/Makefile.am
index 77e75c2..b7039ec 100644
--- a/src/driver/0.24.x/Makefile.am
+++ b/src/driver/0.24.x/Makefile.am
@@ -3,7 +3,7 @@ NULL =
 VERSIONED_VAPI_DIR = $(shell pkg-config libvala-0.24 --variable vapidir)
 
 if HAVE_LIBVALA_0_23_1
-VALAFLAGS = -D VALA_0_23_1
+VALAFLAGS += -D VALA_0_23_1
 endif
 
 AM_CFLAGS = \