util: allow subst to build on systems that do not have utimes()

util: allow subst to build on systems that do not have utimes()

Make subst more portable so it can deal with such oler systems that do
not have utimes().  Note that it is important that subst build
correctly without an autoconf-generated config.h (since that is what
happens on a cross-compile), as well as using whatever features are
available as determined by autoconf when doing a native build.  We
currently assume the presence of utime(), but not utimes() or
futimes().

Signed-off-by: Theodore Ts'o <tytso@mit.edu>