Add gnuXX stds to Objective C.

mesonbuild/compilers/objc.py

diff --git a/mesonbuild/compilers/objc.py b/mesonbuild/compilers/objc.py
index ec8d290..9114f93 100644
--- a/mesonbuild/compilers/objc.py
+++ b/mesonbuild/compilers/objc.py
@@ -90,7 +90,7 @@
         opts.update({
             OptionKey('std', machine=self.for_machine, lang='c'): coredata.UserComboOption(
                 'C language standard to use',
-                ['none', 'c89', 'c99', 'c11', 'c17'],
+                ['none', 'c89', 'c99', 'c11', 'c17', 'gnu99', 'gnu11'],
                 'none',
             )
         })