Require C99's __VA_ARGS__

We have fallback in places for GNU's variadic arguments in macros, and
for static inline functions with variadic arguments as an fallback of
last resort, but going forward we are going to depend on `__VA_ARGS__`
for macros that cannot be re-implemented using a static inline function.

Fixes: #2681

docs/toolchain-requirements.md

@@ -40,12 +40,12 @@ features’ are assumed.  Those are detailed below.
 
 GLib additionally requires Python 3 to build.
 
-Varargs macros
+C99 Varargs macros
 ---
 
-_Not a hard requirement._
+_Hard requirement._
 
-GLib can work with either C99 or GNU style varargs macros.
+GLib requires C99 ``__VA_ARG__`` support for both C and C++ compilers.
 
 Symbol visibility control
 ---

glib/glibconfig.h.in

@@ -118,8 +118,7 @@ typedef unsigned @glib_intptr_type_define@ guintptr;
 
 @glib_vacopy@
 
-@g_have_iso_c_varargs@
-@g_have_iso_cxx_varargs@
+#define G_HAVE_ISO_VARARGS 1
 
 /* gcc-2.95.x supports both gnu style and ISO varargs, but if -ansi
  * is passed ISO vararg support is turned off, and there is no work

meson.build

@@ -1715,11 +1715,8 @@ g_have_iso_c_varargs = cc.compiles('''
     call_a(2,3);
   }''', name : 'ISO C99 varargs macros in C')
 
-if g_have_iso_c_varargs
-  glibconfig_conf.set('g_have_iso_c_varargs', '''
-#ifndef __cplusplus
-# define G_HAVE_ISO_VARARGS 1
-#endif''')
+if not g_have_iso_c_varargs
+  error('GLib requires a C compiler with support for C99 __VA_ARG__ in macros.')
 endif
 
 g_have_iso_cxx_varargs = cxx.compiles('''
@@ -1729,11 +1726,8 @@ g_have_iso_cxx_varargs = cxx.compiles('''
     call_a(2,3);
   }''', name : 'ISO C99 varargs macros in C++')
 
-if g_have_iso_cxx_varargs
-  glibconfig_conf.set('g_have_iso_cxx_varargs', '''
-#ifdef __cplusplus
-# define G_HAVE_ISO_VARARGS 1
-#endif''')
+if not g_have_iso_cxx_varargs
+  error('GLib requires a C++ compiler with support for C99 __VA_ARG__ in macros.')
 endif
 
 g_have_gnuc_varargs = cc.compiles('''