fix-gnulib.pl: Fix dirstamp regression from prior change.

fix-gnulib.pl: Fix dirstamp regression from prior change.

The non-destructive y/// turned out to matter.  Replace it with a
subroutine instead, which can modify a local variable and should
achieve the same results on older perl.

Instead of

  EXTRA_PROGRAMS += lib/sys/gl-dirstamp

We were getting

  EXTRA_PROGRAMS += lib_sys/gl-dirstamp

and this damage was not noticed by any test case.  Add an explicit
check which ensures these lines are output as expected.