Link to install.php needs to respect the WP_SITEURL value, if defined

[JeremyVisser]

When doing ​Install4Free installs, if someone requests that I install WordPress in the root of their domain, I actually put WP in /wordpress/ and put the index.php in the root. By doing that, I'm still doing as they requested, but keeping their public_html folder clean.

Now, I have to say that since WP_HOME and WP_SITEURL got added to wp-config.php this suddenly got a lot faster, but one annoying thing is that when you then access the root of the domain, the greeting message:

It doesn't look like you've installed WP yet. Try running install.php.

...links to the wrong URL. I have to manually navigate to /wordpress/ and click the install.php link from there. At this point, WP is able to read from wp-config.php, so if WP_SITEURL is set by now (which I always do), make it link something like this instead:

if ( defined('WP_SITEURL') )
    echo '<a href="' . WP_SITEURL . '/wp-admin/install.php">install.php</a>';
else // hardcoded:
    echo '<a href="wp-admin/install.php">install.php</a>';

[markjaquith]

(In [5844]) More streamlined install process. Obeys WP_SITEURL if defined. fixes #4687 (props JeremyVisser, Nazgul), see #4685

[JeremyVisser]

Hey cool, I like those changes, Mark!