c - Why is _Generic keyword supported in c99 or c90 modes?

I wrote a simple C program to test the availability of _Generic keyword.

int main() {
    int _Generic;
}

I ran the program with gcc-5.3.1 and clang-3.8.0 compilers on Ubuntu.

Obviously this program generated an error when compiled in the latest c11 standard.

But, when compiled with -std=c90 and -std=c99 flags, it generated an error as well. Whereas, the _Generic keyword was only introduced in c11 standard.

Is it that the -std= flags behave differently? And is there a way to test the pure c90 and c99 standards?

EDIT:

I did run the same program with other identifiers which are not keywords as per c11 standard. Like:

int _Hello;
int _Gener;

And they compiled successfully without any errors or warnings.
This is probably because of 7.1.3, which says

If the program declares or defines an identifier in a
context in which it is reserved (other than as allowed by 7.1.4), or defines a reserved
identifier as a macro name, the behavior is undefined

as said by @Art and @Lundin.