build: Set C standard with CMake and meson

This should add `/std:c11` to MSVC builds which makes sure that the
__STDC_VERSION__ macro is set.

CMakeLists.txt

@@ -10,6 +10,8 @@ endif()
 
 project(libxml2 VERSION ${VERSION} LANGUAGES C)
 
+set(CMAKE_C_STANDARD 11)
+
 include(CheckCSourceCompiles)
 include(CheckFunctionExists)
 include(CheckIncludeFiles)

meson.build

@@ -3,7 +3,11 @@ project(
     'c',
     version: files('VERSION'),
     license: 'MIT',
-    default_options: ['buildtype=debug', 'warning_level=3'],
+    default_options: [
+        'c_std=c11,c99,c89',
+        'buildtype=debug',
+        'warning_level=3',
+    ],
     meson_version: '>= 0.61',
 )
 
@@ -55,6 +59,8 @@ cc = meson.get_compiler('c')
 # global compiler flags
 
 global_args = [
+    '-D_XOPEN_SOURCE=600',
+
     # Enabled by warning_level=3
     # '-pedantic',
     # '-Wall',