* admin/merge-gnulib (GNULIB_MODULES): Add double-slash-root.
Emacs was already using this Gnulib module indirectly, so this
is merely noting that there is now a direct dependency.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.
* src/fileio.c (search_embedded_absfilename):
Use DOUBLE_SLASH_IS_DISTINCT_ROOT instead of (WINDOWSNT || CYGWIN).
Simplify.