meson: explicitly check whether AF_INET6 is available

We're currently not checking whether IPv6 is available before build,
so it just gets silently disabled if AF_INET6 is missing - even
when user wants to enable it explicitly. Adding explicit check into
meson.build, so failing when ipv6 option enabled, but no IPv6 supported.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1539>
(cherry picked from commit bc55a98d62)

meson.build

@@ -281,11 +281,23 @@ if build_xquartz
     build_rootless = true
 endif
 
+# IPv6 support
+have_AF_INET6 = cc.compiles('''
+#ifdef WIN32
+#include <winsock2.h>
+#else
+#include <sys/socket.h>
+#endif
+int main() { int foo = AF_INET6; }''')
 if get_option('ipv6') == 'auto'
-    build_ipv6 = cc.has_function('getaddrinfo')
+    build_ipv6 = cc.has_function('getaddrinfo') and have_AF_INET6
 else
     build_ipv6 = get_option('ipv6') == 'true'
 endif
+if build_ipv6 and not have_AF_INET6
+    error('ipv6 support needs AF_INET6')
+endif
+message('IPv6 support: ' + (build_ipv6 ? 'YES' : 'NO'))
 
 int10 = get_option('int10')
 if int10 == 'auto'