[libglnx/wip/stdc99] build: Use Meson built-in option c_std instead of -std=gnu99

[Simon McVittie <smcv src gnome org>]

commit a4dcfe8d2f81ee8d90570ab5593e3c0947d52371
Author: Simon McVittie <smcv debian org>
Date:   Fri Jul 2 14:19:15 2021 +0100

    build: Use Meson built-in option c_std instead of -std=gnu99
    
    This silences a Meson warning.
    
    Signed-off-by: Simon McVittie <smcv debian org>

 meson.build | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)
---
diff --git a/meson.build b/meson.build
index 74ee36a..cecb060 100644
--- a/meson.build
+++ b/meson.build
@@ -1,7 +1,12 @@
-project('libglnx', 'c')
+project(
+  'libglnx',
+  'c',
+  default_options : [
+    'c_std=gnu99',
+  ],
+)
 
 add_project_arguments('-D_GNU_SOURCE', language: 'c')
-add_project_arguments('-std=gnu99', language: 'c')
 add_project_arguments('-Wno-unused-local-typedefs', language: 'c')
 
 cc = meson.get_compiler('c')