set sane timestamps when generating header symlinks for non-existing headers

set sane timestamps when generating header symlinks for non-existing headers

This is particularly important for QtCore/QtConfig symlink because typically
qconfig.h does not exist when syncqt is run by ./configure. So instead of
assigning a modification timestamp equal to the epoch (00:00 January 1, 1970
GMT.) for the symlink, just use current time().

Gbp-Pq: Name 02_syncqt_sane_timestamp_for_nonexisting_headers.diff