meson: Improve PTRACE_O_EXITKILL presence check

The PTRACE_O_EXITKILL symbol in sys/ptrace.h is an enum member, not
a macro. The #ifdef check added to the GSubprocess test-case in
272ec5dbca will not detect it.

Use cc.has_header_symbol() to properly detect it. According to the
documentation: "Symbols here include function, variable, #define,
type definition, etc.".

Fixes: 272ec5dbca
Closes: https://gitlab.gnome.org/GNOME/glib/-/issues/3156

gio/tests/gsubprocess.c

@@ -2006,7 +2006,7 @@ trace_children (pid_t main_child)
   g_assert_no_errno (waitpid (main_child, &wstatus, 0));
   g_assert_no_errno (ptrace (PTRACE_SETOPTIONS, main_child, NULL,
                              (PTRACE_O_TRACEFORK |
-#ifdef PTRACE_O_EXITKILL
+#ifdef HAVE_PTRACE_O_EXITKILL
                               PTRACE_O_EXITKILL |
 #endif
                               PTRACE_O_TRACEVFORK |

meson.build

@@ -2440,6 +2440,10 @@ with open(output, "w") as f:
   enable_dtrace = true
 endif
 
+if cc.has_header_symbol('sys/ptrace.h', 'PTRACE_O_EXITKILL')
+  glib_conf.set('HAVE_PTRACE_O_EXITKILL', 1)
+endif
+
 # systemtap
 want_systemtap = get_option('systemtap')
 enable_systemtap = false