Set executable stack flag when linking

This is required on some modern hardneded Linux systems to allow
to dlopen executable objects (load plugins).

https://discourse.ardour.org/t/tls-1295-lea-so-linux-vers-doesnt-work/111778/20?u=x42

libs/vfork/wscript

@@ -1,5 +1,6 @@
 #!/usr/bin/env python
 import os
+import sys
 
 def options(opt):
     pass
@@ -10,6 +11,7 @@ def configure(conf):
 def build(bld):
     if bld.env['build_target'] == 'mingw':
         return
+
     obj = bld (features = 'c cprogram')
     obj.source = 'exec_wrapper.c'
     obj.target = 'ardour-exec-wrapper'
@@ -18,3 +20,8 @@ def build(bld):
             '_POSIX_SOURCE',
             '_XOPEN_SOURCE=500',
     ]
+
+    if sys.platform == 'darwin':
+        pass
+    else:
+        obj.linkflags = ' -zexecstack'