meson: Define G_OS_DARWIN when compiling under OSX or iOS

It has enough differences to expose it as an unix-subtype.

docs/reference/glib/glib-overrides.txt

@@ -196,6 +196,11 @@ GCond *cond
 
 # Definitions for different operating systems
 
+<MACRO>
+<NAME>G_OS_DARWIN</NAME>
+#define G_OS_DARWIN
+</MACRO>
+
 <MACRO>
 <NAME>G_OS_UNIX</NAME>
 #define G_OS_UNIX

docs/reference/glib/glib-sections.txt.in

@@ -140,6 +140,7 @@ GLIB_VERSION_PREV_STABLE
 <TITLE>Standard Macros</TITLE>
 <FILE>macros</FILE>
 <SUBSECTION>
+G_OS_DARWIN
 G_OS_WIN32
 G_OS_UNIX
 

glib/docs.c

@@ -1647,6 +1647,17 @@
  * These macros provide a few commonly-used features.
  */
 
+/**
+ * G_OS_DARWIN:
+ *
+ * This macro is defined only on Apple operating systems (macOS or iOS).
+ * So you can bracket Apple-specific code in `\#ifdef G_OS_DARWIN`.
+ *
+ * Note that %G_OS_UNIX is also set.
+ *
+ * Since: 2.76
+ */
+
 /**
  * G_OS_WIN32:
  *

glib/tests/macros.c

@@ -20,8 +20,19 @@
  * Author: Philip Withnall <withnall@endlessm.com>
  */
 
+#include "config.h"
+
 #include <glib.h>
 
+#if defined (__APPLE__) || defined (HAVE_COCOA) || defined (HAVE_CARBON)
+# ifndef G_OS_UNIX
+  G_STATIC_ASSERT (FALSE);
+# endif
+# ifndef G_OS_DARWIN
+  G_STATIC_ASSERT (FALSE);
+# endif
+#endif
+
 /* Test that G_STATIC_ASSERT_EXPR can be used as an expression */
 static void
 test_assert_static (void)

meson.build

@@ -235,6 +235,9 @@ if host_system == 'windows'
 elif host_system == 'cygwin'
   glib_os = '''#define G_OS_UNIX
 #define G_WITH_CYGWIN'''
+elif host_system in ['darwin', 'ios']
+  glib_os = '''#define G_OS_UNIX
+#define G_OS_DARWIN'''
 else
   glib_os = '#define G_OS_UNIX'
 endif