Support for GNU GCC nested functions

(...or better not!)

I stumbled over this concession to Pascal programmers:
https://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html

Test case:
https://github.com/JoliOS/grub2/blob/master/util/grub-fstest.c#L211

Note that this feature is not used in the original grub code:
http://git.savannah.gnu.org/cgit/grub.git/tree/util/grub-fstest.c#n240
