Don't encode the current timestamp.

This affects the output of `perl -V`, specifically the message "Compiled
at [...]".

The 'cf_time' and 'cf_by' values show up in 'config.h' and
in 'Config_heavy.pl'.

Use the output of 'uname -s' instead of 'uname -a' to avoid recording
the kernel version ('uname -o' leads to directory names like
'x86_64-gnulinux' instead of 'x86_64-linux', which might cause breakage
down the road.)

diff --git a/perl.c b/perl.c
index 228a0d8..ed38313 100644
--- a/perl.c
+++ b/perl.c
@@ -1825,6 +1825,7 @@ S_Internals_V(pTHX_ CV *cv)
     PUSHs(Perl_newSVpvn_flags(aTHX_ non_bincompat_options,
 			      sizeof(non_bincompat_options) - 1, SVs_TEMP));
 
+#define PERL_BUILD_DATE "Jan  1 1970 00:00:00"
 #ifndef PERL_BUILD_DATE
 #  ifdef __DATE__
 #    ifdef __TIME__

--- a/Configure	1970-01-01 01:00:00.000000000 +0100
+++ b/Configure	2016-10-01 14:47:20.017319739 +0200
@@ -3276,7 +3276,7 @@ $eunicefix tr
 : Try to determine whether config.sh was made on this system
 case "$config_sh" in
 '')
-myuname=`$uname -a 2>/dev/null`
+myuname=`$uname -s 2>/dev/null`
 $test -z "$myuname" && myuname=`hostname 2>/dev/null`
 # Downcase everything to avoid ambiguity.
 # Remove slashes and single quotes so we can use parts of this in
@@ -3845,10 +3845,10 @@
 . ./posthint.sh
 
 : who configured the system
-cf_time=`LC_ALL=C; LANGUAGE=C; export LC_ALL; export LANGUAGE; $date 2>&1`
+cf_time="1970-01-01"
 case "$cf_by" in
 "")
-	cf_by=`(logname) 2>/dev/null`
+	cf_by="guix"
 	case "$cf_by" in
 	"")
 		cf_by=`(whoami) 2>/dev/null`