[gnome-settings-daemon] build: Require a C99 compiler

[Richard Hughes <rhughes src gnome org>]

commit 3ca5a8eea2e17616aab3473d71fd9401f42ec3a5
Author: Richard Hughes <richard hughsie com>
Date:   Thu Feb 2 13:40:23 2017 +0000

    build: Require a C99 compiler
    
    It's 2017 now and it would be great to be able to use newer compiler features
    like defining a variable anywhere we want.

 configure.ac |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index dc815eb..6e0d0dc 100644
--- a/configure.ac
+++ b/configure.ac
@@ -20,6 +20,7 @@ AC_SUBST(GSD_API_VERSION)
 AC_STDC_HEADERS
 AC_PROG_CXX
 AM_PROG_CC_C_O
+AC_PROG_CC_C99
 AC_PROG_LIBTOOL
 
 AC_HEADER_STDC