$ echo "long long x;" > z.c
$ gcc --std=c99 -pedantic -Wall -c z.c
$ gcc --std=c90 -pedantic -Wall -c z.c
cc1: error: unrecognized command line option "-std=c90"
$ gcc --std=c89 -pedantic -Wall -c z.c
z.c:1: warning: ISO C90 does not support `long long'

Hmm. Nothing like clear consensus on standards nomenclature.