Checking __STDC_VERSION__ rather than __STRICT_ANSI__

Checking __STDC_VERSION__ rather than __STRICT_ANSI__

`__STRICT_ANSI__` is a gnuish flag macro that indicates if `-ansi`
was given on the command line.  To check the C version, it's better
to check the macro `__STDC_VERSION__`.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/19197)