That's a GNU extension that everyone relies so much on it's basically akin to a standard. You get a warning from Clang (or an error from GCC) if you ask for a more "standard" interpretation of the source:

    $ clang -o cs cs.c -Wall -std=c11 -pedantic
    cs.c:6:6: warning: expression is not an integer constant expression; folding it to a constant is a GNU extension [-Wgnu-folding-constant]
        A = X,
    $ gcc -o cs cs.c -Wall -std=c11
    cs.c:6:9: error: enumerator value for ‘A’ is not an integer constant
        6 |         A = X,
          |         ^

In general, that's illegal ISO C and should always be rejected, but as you see that's not usually the case.