patch 8.2.0910: Vim is not reproducibly buildable

Problem:    Vim is not reproducibly buildable.
Solution:   Use the $SOURCE_DATE_EPOCH environment variable in configure.
            (James McCoy, closes #513)  Give a warning about using it.
diff --git a/src/auto/configure b/src/auto/configure
index 516e45c..4c3ff32 100755
--- a/src/auto/configure
+++ b/src/auto/configure
@@ -721,6 +721,7 @@
 XCODE_SELECT
 CPP_MM
 CROSS_COMPILING
+BUILD_DATE_MSG
 STRIP
 AWK
 FGREP
@@ -4455,6 +4456,17 @@
   fi
 fi
 
+if test -n "$SOURCE_DATE_EPOCH"; then
+  DATE_FMT="%b %d %Y %H:%M:%S"
+  BUILD_DATE=$(LC_ALL=C date -u -d "@$SOURCE_DATE_EPOCH" "+$DATE_FMT" 2>/dev/null || LC_ALL=C date -u -r "$SOURCE_DATE_EPOCH" "+$DATE_FMT" 2>/dev/null || LC_ALL=C date -u "+$DATE_FMT")
+  cat >>confdefs.h <<_ACEOF
+#define BUILD_DATE "$BUILD_DATE"
+_ACEOF
+
+  BUILD_DATE_MSG=-"echo -e '=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=\nNOTE: build date/time is fixed: $BUILD_DATE\n=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-='"
+
+fi
+
 
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking --enable-fail-if-missing argument" >&5
 $as_echo_n "checking --enable-fail-if-missing argument... " >&6; }