provide SOURCE_DATE_EPOCH when building from source

in preparation of adding options to use use stable timestamps in created
zip files/templates and similar.

also move the block that sets LIBO_THIS_YEAR to the section where
configure switches to SRCDIR to make it work in builddir != srcdir case

Change-Id: I286d7765277cde9d8449de375f522df7c33a6222
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/207015
Tested-by: Jenkins
Reviewed-by: René Engelhard <rene@debian.org>

config_host.mk.in

@@ -643,6 +643,8 @@ export SKIA_DISABLE_VMA_USE_STL_SHARED_MUTEX=@SKIA_DISABLE_VMA_USE_STL_SHARED_MU
 export SKIA_GPU=@SKIA_GPU@
 export SOLARINC=@SOLARINC@
 export SORT=@SORT@
+# conditional assignment, to allow easy override by environment variable vs explicit argument
+SOURCE_DATE_EPOCH?=@SOURCE_DATE_EPOCH@
 export SPLIT_APP_MODULES=@SPLIT_APP_MODULES@
 export SPLIT_OPT_FEATURES=@SPLIT_OPT_FEATURES@
 export SRCDIR=@SRC_ROOT@

configure.ac

@@ -606,10 +606,6 @@ AC_DEFINE_UNQUOTED(LIBO_VERSION_MINOR,$LIBO_VERSION_MINOR)
 AC_DEFINE_UNQUOTED(LIBO_VERSION_MICRO,$LIBO_VERSION_MICRO)
 AC_DEFINE_UNQUOTED(LIBO_VERSION_PATCH,$LIBO_VERSION_PATCH)
 
-git_date=`git log -1 --pretty=format:"%cd" --date=format:'%Y' 2>/dev/null`
-LIBO_THIS_YEAR=${git_date:-2026}
-AC_DEFINE_UNQUOTED(LIBO_THIS_YEAR,$LIBO_THIS_YEAR)
-
 dnl ===================================================================
 dnl Product version
 dnl ===================================================================
@@ -624,6 +620,14 @@ AC_PATH_PROG(GREP, grep)
 
 BUILDDIR=`pwd`
 cd $srcdir
+
+git_date=`git log -1 --pretty=format:"%cd" --date=format:'%Y' 2>/dev/null`
+LIBO_THIS_YEAR=${git_date:-2026}
+AC_DEFINE_UNQUOTED(LIBO_THIS_YEAR,$LIBO_THIS_YEAR)
+# commit-time in unix epoch timestamp format in prep for reproducible builds
+SOURCE_DATE_EPOCH=`git log -1 --pretty=%ct 2>/dev/null`
+AC_SUBST(SOURCE_DATE_EPOCH)
+
 SRC_ROOT=`pwd`
 cd $BUILDDIR
 x_Cygwin=[\#]