Prevent intl/plural.c being regenerated when building included libintl

Prevent intl/plural.c being regenerated when building included libintl

The Makefile included with libintl contains a rule to regenerate plural.c from
plural.y if necessary (it's not necessary unless we'd develop gettext itself).
Extracting files from a tarball or autopoint copying them in alphabetical order
makes plural.c slightly older than plural.y (because 'c' comes before 'y') and
that triggers the regeneration rule in the Makefile. In most cases that causes
compilation errors.

Therefore we try our best to prevent it from being regenerated by setting the
last modification time of plural.c to the current time (which must already be
older than the last modification time of plural.y).