Make lib/dns/gen.c compatible with reproducible builds.

The gen.c will now use SOURCE_DATE_EPOCH[1] if found in environment
to make the build more reproducible build friendly.

1. https://reproducible-builds.org/specs/source-date-epoch/
39 jobs for !1758 with ondrej/reproducible-build in 14 minutes and 15 seconds (queued for 3 seconds)
detached
Status Job ID Name Coverage
  Precheck
passed #226975
amd64 linux
🐞:sid:amd64

00:00:13

passed #226974
amd64 linux
misc:sid:amd64

00:00:17

 
  Build
passed #226986
amd64 linux
asan:sid:amd64

00:04:55

passed #226987
amd64 linux
clang:stretch:amd64

00:01:45

passed #226988
i386 linux
clang:stretch:i386

00:02:34

passed #226976
amd64 linux
docs:sid:amd64

00:00:51

passed #226985
amd64 linux
gcc:bionic:amd64

00:02:44

passed #226977
amd64 linux
gcc:centos6:amd64

00:02:05

passed #226978
amd64 linux
gcc:centos7:amd64

00:02:28

passed #226983
amd64 linux
gcc:fedora29:amd64

00:02:39

passed #226979
amd64 linux
gcc:jessie:amd64

00:01:28

passed #226981
amd64 linux
gcc:sid:amd64

00:02:25

passed #226982
i386 linux
gcc:sid:i386

00:03:04

passed #226980
amd64 linux
gcc:stretch:amd64

00:02:44

passed #226984
amd64 linux
gcc:xenial:amd64

00:02:20

passed #226989
amd64 linux
pkcs11:sid:amd64

00:02:38

 
  Test
passed #227008
amd64 linux
system:asan:sid:amd64

00:09:02

passed #227006
amd64 linux
system:gcc:bionic:amd64

00:07:58

passed #226990
amd64 linux
system:gcc:centos6:amd64

00:06:25

passed #226992
amd64 linux
system:gcc:centos7:amd64

00:06:45

passed #227002
amd64 linux
system:gcc:fedora29:amd64

00:06:49

passed #226994
amd64 linux
system:gcc:jessie:amd64

00:06:58

passed #226998
amd64 linux
system:gcc:sid:amd64

00:06:39

passed #227000
i386 linux
system:gcc:sid:i386

00:07:13

passed #226996
amd64 linux
system:gcc:stretch:amd64

00:07:05

passed #227004
amd64 linux
system:gcc:xenial:amd64

00:07:58

passed #227011
amd64 linux
system:pkcs11:sid:amd64

00:06:51

passed #227009
amd64 linux
unit:asan:sid:amd64

00:00:57

passed #227010
amd64 linux
unit:clang:stretch:amd64

00:00:43

passed #227007
amd64 linux
unit:gcc:bionic:amd64

00:00:48

passed #226991
amd64 linux
unit:gcc:centos6:amd64

00:00:48

passed #226993
amd64 linux
unit:gcc:centos7:amd64

00:00:48

passed #227003
amd64 linux
unit:gcc:fedora29:amd64

00:00:50

passed #226995
amd64 linux
unit:gcc:jessie:amd64

00:00:30

passed #226999
amd64 linux
unit:gcc:sid:amd64

00:00:45

passed #227001
i386 linux
unit:gcc:sid:i386

00:00:49

passed #226997
amd64 linux
unit:gcc:stretch:amd64

00:00:49

passed #227005
amd64 linux
unit:gcc:xenial:amd64

00:00:54

passed #227012
amd64 linux
unit:pkcs11:sid:amd64

00:00:52