libproc.h isn't available on older systems

--- meson.build.orig	2023-05-13 23:04:36.000000000 -0400
+++ meson.build	2023-05-13 23:04:51.000000000 -0400
@@ -350,6 +350,7 @@
   'fstab.h',
   'grp.h',
   'inttypes.h',
+  'libproc.h',
   'limits.h',
   'locale.h',
   'mach/mach_time.h',
--- glib/gspawn.c.orig	2023-05-13 23:05:53.000000000 -0400
+++ glib/gspawn.c	2023-05-13 23:06:23.000000000 -0400
@@ -70,7 +70,7 @@
 #include "glibintl.h"
 #include "glib-unix.h"
 
-#ifdef __APPLE__
+#ifdef HAVE_LIBPROC_H
 #include <libproc.h>
 #include <sys/proc_info.h>
 #endif
@@ -1544,7 +1544,7 @@
   if (open_max < 0)
     open_max = 4096;
 
-#if defined(__APPLE__)
+#if defined(HAVE_LIBPROC_H)
   /* proc_pidinfo isn't documented as async-signal-safe but looking at the implementation
    * in the darwin tree here:
    *