Do not introduce timestamps in PNG files (for icons).
See <https://wiki.debian.org/ReproducibleBuilds/TimestampsInPNG>.

--- awesome-3.4.15/CMakeLists.txt	2013-02-11 14:50:21.000000000 +0100
+++ awesome-3.4.15/CMakeLists.txt	2016-06-16 13:59:30.528125439 +0200
@@ -295,7 +295,7 @@ macro(a_icon_convert match replacement i
         set(ALL_ICONS ${ALL_ICONS} ${output})
 
         add_custom_command(
-            COMMAND ${CONVERT_EXECUTABLE} ${input} -strip ${ARGN} ${output}
+            COMMAND ${CONVERT_EXECUTABLE} +set date:create +set date:modify -define png:exclude-chunk=time ${input} -strip ${ARGN} ${output}
             OUTPUT  ${output}
             DEPENDS ${input}
             VERBATIM)
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<tr><td><span title='2017-07-22 23:48:58 +0200'>2017-07-22</span></td><td><a href='/guix/commit/gnu/packages/medical.scm?id=a68a83f0156d17dfdfb95557e5cf36a1a024dfc7'>gnu: Add openmolar.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>