build: Use timestamps file based on last commit date instead of host date

.gitignore

@@ -30,6 +30,7 @@
 !/common/font.bin
 /configure
 /configure.ac.save
+/timestamps
 /build-aux
 /aclocal.m4
 /*~

GNUmakefile.in

@@ -335,7 +335,7 @@ distclean: clean
 
 .PHONY: maintainer-clean
 maintainer-clean: distclean
-	cd '$(call SHESCAPE,$(SRCDIR))' && rm -rf common/flanterm common/lib/stb_image.h decompressor/tinf tinf stb freestanding-headers common/cc-runtime decompressor/cc-runtime limine-efi configure build-aux *'~' autom4te.cache aclocal.m4 *.tar*
+	cd '$(call SHESCAPE,$(SRCDIR))' && rm -rf common/flanterm common/lib/stb_image.h decompressor/tinf tinf stb freestanding-headers common/cc-runtime decompressor/cc-runtime limine-efi configure timestamps build-aux *'~' autom4te.cache aclocal.m4 *.tar*
 
 .PHONY: common-uefi-x86-64
 common-uefi-x86-64:

bootstrap

@@ -66,6 +66,20 @@ if ! [ -f version ]; then
     rm -f common/lib/stb_image.h.orig
 fi
 
+# Create timestamps file
+if git log -1 >/dev/null 2>&1; then
+    cat >timestamps <<EOF
+REGEN_DATE="$(git log -1 --pretty=%cd --date='format:%B %Y')"
+SOURCE_DATE_EPOCH="$(git log -1 --pretty=%ct)"
+SOURCE_DATE_EPOCH_TOUCH="$(git log -1 --pretty=%cI | sed 's/-//g;s/T//g;s/://g;s/[0-9][0-9]+.*//g')"
+EOF
+fi
+
+if ! test -f timestamps; then
+    echo "error: Not a Git repository and 'timestamps' file missing."
+    exit 1
+fi
+
 for auxfile in $AUXFILES; do
     rm -f build-aux/$auxfile
 done

configure.ac

@@ -10,13 +10,10 @@ BUILDDIR="$(pwd -P)"
 AC_SUBST([SRCDIR])
 AC_SUBST([BUILDDIR])
 
-REGEN_DATE="m4_esyscmd([date '+%B %Y' | tr -d '\n'])"
+. "$SRCDIR"/timestamps
+
 AC_SUBST([REGEN_DATE])
-
-SOURCE_DATE_EPOCH="m4_esyscmd([if git log -1 >/dev/null 2>&1; then git log -1 --pretty=%ct | tr -d '\n'; else printf 1546300800; fi])"
 AC_SUBST([SOURCE_DATE_EPOCH])
-
-SOURCE_DATE_EPOCH_TOUCH="m4_esyscmd([if git log -1 >/dev/null 2>&1; then git log -1 --pretty=%cI | sed 's/-//g;s/T//g;s/://g;s/[0-9][0-9]+.*//g' | tr -d '\n'; else printf 201901010000; fi])"
 AC_SUBST([SOURCE_DATE_EPOCH_TOUCH])
 
 AC_CANONICAL_HOST