[glib] configure.ac: add -Wduplicated-branches

[Philip Withnall <pwithnall src gnome org>]

commit aef0e694eb9bc853d957628bc541618cbdd35261
Author: Ernestas Kulik <ernestask gnome org>
Date:   Fri Feb 16 19:12:33 2018 +0200

    configure.ac: add -Wduplicated-branches
    
    https://bugzilla.gnome.org/show_bug.cgi?id=793399

 configure.ac | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/configure.ac b/configure.ac
index 5dcdb79a7..de97a8f68 100644
--- a/configure.ac
+++ b/configure.ac
@@ -3486,7 +3486,8 @@ AC_ARG_ENABLE(compile-warnings,
                              enable_compile_warnings=yes)
 AS_IF([test "x$enable_compile_warnings" != xno], [
   CC_CHECK_FLAGS_APPEND([GLIB_WARN_CFLAGS], [CFLAGS], [\
-   -Wall -Wstrict-prototypes -Werror=declaration-after-statement \
+   -Wall -Wstrict-prototypes -Wduplicated-branches \
+   -Werror=declaration-after-statement \
    -Werror=missing-prototypes -Werror=implicit-function-declaration \
    -Werror=pointer-arith -Werror=init-self -Werror=format-security \
    -Werror=format=2 -Werror=missing-include-dirs])