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