__TIMESTAMP_UNIXEPOCH__ instead of useless __TIMESTAMP__?

[Walter Bright]

On 1/24/2018 11:18 AM, Timothee Cour wrote:
> __TIMESTAMP__ is pretty useless:
> `string literal of the date and time of compilation "www mmm dd hh:mm:ss yyyy"`
> eg:Wed Jan 24 11:03:56 2018
> which is a weird non-standard format not understood by std.datetime.

It's the format emitted by the Standard C library function asctime():

     http://pubs.opengroup.org/onlinepubs/009695399/functions/asctime.html

> __DATE__ and __TIME__ are also pretty useless.

These also match the format of the Standard C preprocessor macros __DATE__ and 
__TIME__.