Allow to override build date with SOURCE_DATE_EPOCH

Allow to override build date with SOURCE_DATE_EPOCH in order to make builds reproducible. See https://reproducible-builds.org/ for why this is good and https://reproducible-builds.org/specs/source-date-epoch/ for the definition of this variable.

Without this patch, there were such diffs:

--- old//usr/share/doc/packages/vtk-openmpi2-8.2/doxygen/html/mc_vtk_v_0.html   2019-06-05 23:51:08.144823608 +0000
+++ new//usr/share/doc/packages/vtk-openmpi2-8.2/doxygen/html/mc_vtk_v_0.html   2019-06-05 23:51:08.144823608 +0000
@@ -43,7 +43,7 @@
 </div><!--header-->
 <div class="contents">
 <div class="textblock"><ul>
-<li>2533 implementation file(s) returning 99 word(s) for 140 classe(es) on Wed Jun 5 13:22:03 2019</li>
+<li>2533 implementation file(s) returning 99 word(s) for 140 classe(es) on Sat Jul 8 05:14:09 2034</li>
 <li>0 file(s) updated</li>

--- old//usr/share/doc/packages/vtk-openmpi2-8.2/doxygen/html/c2_vtk_e_0.html   2019-06-05 23:51:03.176807768 +0000
+++ new//usr/share/doc/packages/vtk-openmpi2-8.2/doxygen/html/c2_vtk_e_0.html   2019-06-05 23:51:03.176807768 +0000
@@ -43,7 +43,7 @@
 </div><!--header-->
 <div class="contents">
 <div class="textblock"><ul>
-<li>425 class(es) in 363 file(s) from directories matching <code>^Examples$</code> on Wed Jun 5 13:22:06 2019</li>
+<li>425 class(es) in 363 file(s) from directories matching <code>^Examples$</code> on Sat Jul 8 05:14:12 2034</li>
 <li>3 parser(s) : [C++, Python, Java]</li>

--- old//usr/share/doc/packages/vtk-openmpi2-8.2/doxygen/html/idx_vtk_0.html    2019-06-05 23:51:05.320814604 +0000
+++ new//usr/share/doc/packages/vtk-openmpi2-8.2/doxygen/html/idx_vtk_0.html    2019-06-05 23:51:05.320814604 +0000
@@ -43,7 +43,7 @@
 </div><!--header-->
 <div class="contents">
 <div class="textblock"><ul>
-<li>2837 file(s) indexed by 8078 word(s) on Wed Jun 5 13:22:05 2019</li>
+<li>2837 file(s) indexed by 8078 word(s) on Sat Jul 8 05:14:10 2034</li>
 <li>max limit is 10 xref(s) per word</li>
 </ul>

So all 3 hunks are actually needed.

Topic-rename: reproducible-doxygen-source-date-epoch