GCC, Objective-C and C99

[Alan Third]

I've pushed a new test into scratch/ns-gitlab-ci that runs a build
against GNUstep on emba, however it's failing. I get this error:

nsterm.m: In function '-[EmacsView insertText:]':
nsterm.m:6636:3: error: 'for' loop initial declarations are only allowed in C99 
or C11 mode
   for (NSUInteger i = 0; i < len; i++)
   ^~~

Now, as far as I can tell all I have to do is add -std=c99 when
compiling Objective C, but I'm really not sure where to add it. In a
test I added it to CFLAGS when I configured, and that worked, but that
doesn't seem like a proper fix.

What's really odd is that I never had this problem when I built on a
VM based off Debian Jessie, but I now do with the Debian Stretch
Docker image. I guess something's changed with GCC.
-- 
Alan Third