build: don't recompress existing files on install
build: don't recompress existing files on install

We do `gzip` on installation of our compressed web resources in order to
avoid compressing them at build time.  In the case of repeatedly
installing into the same DESTDIR this wastes a bit of time.

If DESTDIR is set, check if the installed file is already newer than the
one we're installing, and skip the compression if it is.

This drops the time needed for an incremental `make install` into a
`DESTDIR` from about 1.85s to about 0.6s.

In the case of doing a naive `make install` then there is no change to
current behaviour.

martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit