It compiles and runs with gcc -std=c89 and gcc -std=c99, so even if it's not a true C89 program, it's a compilable GNUC89 program.
$ gcc -std=c99 -pedantic /tmp/main.c -o /tmp/main
/tmp/main.c:1:1: warning: data definition has no type
or storage class [enabled by default]
/tmp/main.c:1:1: warning: type defaults to ‘int’ in
declaration of ‘main’ [enabled by default]
/tmp/main.c:1:1: warning: ‘main’ is usually a function
[-Wmain]
$ /tmp/main
Segmentation fault (core dumped)
gcc -nostdlib -std=c89 a.c -o a
a.c:1: warning: data definition has no type or storage class
Undefined symbols for architecture x86_64:
"start", referenced from:
-u command line option
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status`
