Gitweb: add support for minifying gitweb.css

Gitweb: add support for minifying gitweb.css

The build system added support minifying gitweb.js through a
JavaScript minifier, but most minifiers come with support for
minifying CSS files as well, so we should use it if we can.

This patch will add the same facilities to gitweb.css that
gitweb.js has for minification. That does not mean that they
will use the same minifier though, as it is not safe to assume
that all JavaScript minifiers will also minify CSS files.

This patch also adds the GITWEB_PROGRAMS variable to the Makefile
to keep a list of potential gitweb dependencies separate from
OTHER_PROGRAMS when we need to know just the gitweb dependencies.

Though the bandwidth savings will not be as dramatic as with
the JavaScript minifier, every byte saved is important.

Signed-off-by: Mark Rada <marada@uwaterloo.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>