Make installer add symlink from `3delight-major.minor` to current install

Rationale: I am too lazy to update my .zshrc ($DELIGHT) every time I update the renderer.

For Houdini I dont have to touch $HFS in my .zshrc until I switch at least minor version. When I install a new version, a symlink from /opt/hfs-major.minor is created/updated automatically by the installer to point to /opt/hfs-major.minor.patch.

Similarly, I suggest creating /usr/local/3delight-major.minor pointing to /usr/local/3delight-major.minor.patch. via the install script.

Here is a diff that adds this to the script:

4a5,6
> $version =~ '^(\d+\.)?(\d+)?';
> $version_no_patch = $&;
18a21
> $latest_dir = $prefix."/3delight-$version_no_patch";
42a46,48
> 
> # Update symlink
> system( "ln -s -f $installdir $latest_dir" );