GCC doesn't set __STDC_VERSION__ for older versions. It is verified to
be set by default on 5.3.0.
Older versions set __INT64_MAX__ though (oldest version tested is
4.5.4).
If compilation with older GCC versions is required, set -std=c99. Even
3.4.6 is supported with this setting.
move some system-specific seed-related stuff from hacklib.c to
a system-specific source file and #define SYS_RANDOM_SEED to
utilize it during build.
Windows changes for random seed generation using
crypto next gen (CNG) api routines.
Corresponding vms changes due to disentangling of VMS and
unix when the unix seed bits got moved (untested).
