tests: skip objc nsstring test if gnustep is not installed

test cases/objc/2 nsstring/meson.build

@@ -5,7 +5,10 @@ if host_machine.system() == 'darwin'
 elif host_machine.system() == 'cygwin'
   error('MESON_SKIP_TEST GNUstep is not packaged for Cygwin.')
 else
-  dep = dependency('gnustep')
+  dep = dependency('gnustep', required : false)
+  if not dep.found()
+    error('MESON_SKIP_TEST: GNUstep is not installed')
+  endif
   if host_machine.system() == 'linux' and meson.get_compiler('objc').get_id() == 'clang'
     error('MESON_SKIP_TEST: GNUstep is broken on Linux with Clang')
   endif