Always provide the same date and time in 'Py_GetBuildInfo'.
This is the information shown at the REPL and in 'sys.version'.
We cannot pass it in CPPFLAGS due to whitespace in the DATE string.

--- a/Modules/getbuildinfo.c
+++ b/Modules/getbuildinfo.c
@@ -4,6 +4,10 @@
 #include <stdio.h>
 #endif
 
+/* Deterministic date and time.  */
+#define DATE "Jan  1 1970"
+#define TIME "00:00:01"
+
 #ifndef DATE
 #ifdef __DATE__
 #define DATE __DATE__
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<tr><td><span title='2017-04-16 00:48:08 +0200'>2017-04-16</span></td><td><a href='/guix/commit/gnu/services/cups.scm?id=3d3c56500843b5573ba6419db5e66075fb8ac8ef'>services: Add a default value to various service types.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-03-12 19:09:32 +0100'>2017-03-12</span></td><td><a href='/guix/commit/gnu/services/cups.scm?id=720cb10c15a4606fe0dc3511db4fef325f3d9dc6'>services: Move configuration functions that shouldn't be factorized.</a><span class='msg-avail'>...</span></td><td>Clément Lassieur</td></tr>
<tr><td><span title='2017-01-25 14:05:37 +0100'>2017-01-25</span></td><td><a href='/guix/commit/gnu/services/cups.scm?id=e57bd0bed86caac304e5f37c5653e6b13858c7c5'>services: Fix 'mkdir-p' in activation scripts.</a><span class='msg-avail'>...</span></td><td>Clément Lassieur</td></tr>
<tr><td><span title='2016-12-01 11:03:20 +0100'>2016-12-01</span></td><td><a href='/guix/commit/gnu/services/cups.scm?id=4ce8860df9c053e3e111f04adcbb636728954251'>services: cups: Follow symlinks when installing extensions.</a><span class='msg-avail'>...</span></td><td>Andy Patterson</td></tr>
<tr><td><span title='2016-11-26 19:46:01 +0100'>2016-11-26</span></td><td><a href='/guix/commit/gnu/services/cups.scm?id=5305ed20027a32ff1221cac6a131849852e807ba'>services: Factorize configuration abstraction.</a><span class='msg-avail'>...</span></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2016-10-13 11:37:25 +0200'>2016-10-13</span></td><td><a href='/guix/commit/gnu/services/cups.scm?id=f2ec23d18e50eef3e4f574a3e6498d8964e4e1a3'>gnu: Add CUPS service.</a><span class='msg-avail'>...</span></td><td>Andy Wingo</td></tr>