Improve reproducible builds.

Use a SOURCE_DATA_EPOCH friendly date.
Suggested by @fasterit in #476.

configure.ac

@@ -4,7 +4,8 @@
 AC_PREREQ(2.65)
 AC_INIT([htop],[2.0.1],[hisham@gobolinux.org])
 
-year=$(date +%Y)
+SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH:-$(date +%s)}"
+year=$(date -u -d "@$SOURCE_DATE_EPOCH" "+%Y" 2>/dev/null || date -u -r "$SOURCE_DATE_EPOCH" "+%Y" 2>/dev/null || date -u "+%Y")
 
 # The following two lines are required by hwloc scripts
 AC_USE_SYSTEM_EXTENSIONS