Add configure.ac / GNU Autotools example for using SOURCE_DATE_EPOCH.

The above will work with either GNU or BSD date, and fallback to ignore `SOURCE_DATE_EPOCH` if both fails.

### GNU Autotools / `configure.ac`

```make

BUILD_DATE=m4_esyscmd([date --utc --date="@${SOURCE_DATE_EPOCH:-$(date +%s)}" %Y-%m-%d])

AC_SUBST(BUILD_DATE)

```

or:

```make

DATE_FMT="%Y-%m-%d"

BUILD_DATE=`date $DATE_FMT`

if test "x$SOURCE_DATE_EPOCH" != "x"; then

    BUILD_DATE=`date -u -d "@$SOURCE_DATE_EPOCH" $DATE_FMT 2>/dev/null || date -u -r $SOURCE_DATE_EPOCH $DATE_FMT || date -u $DATE_FMT`

fi

AC_SUBST(BUILD_DATE)

```

### CMake

```cmake