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.

OpenRGB.pro

@@ -24,7 +24,7 @@ TEMPLATE    = app
 # Automatically generated build information                             #
 #-----------------------------------------------------------------------#
 win32:BUILDDATE = $$system(date /t)
-unix:BUILDDATE  = $$system(date -R)
+unix:BUILDDATE  = $$system(date -R -d "@${SOURCE_DATE_EPOCH:-$(date +%s)}")
 GIT_COMMIT_ID   = $$system(git --git-dir $$_PRO_FILE_PWD_/.git --work-tree $$_PRO_FILE_PWD_ rev-parse HEAD)
 GIT_COMMIT_DATE = $$system(git --git-dir $$_PRO_FILE_PWD_/.git --work-tree $$_PRO_FILE_PWD_ show -s --format=%ci HEAD)
 GIT_BRANCH      = $$system(git --git-dir $$_PRO_FILE_PWD_/.git --work-tree $$_PRO_FILE_PWD_ rev-parse --abbrev-ref HEAD)