[Bug] make gitweb with GITWEB_JS set fails to build

[Michal Vasilek <michal.vasilek@nic.cz>]

What did you do before the bug happened? (Steps to reproduce your issue)

I ran make gitweb with the GITWEB_JS variable set on git 2.39.2, 
specifically:

     make gitweb GITWEB_JS=/www/gitweb.js


What did you expect to happen? (Expected behavior)

I expected the build to pass and that the GITWEB_JS (or GITWEB_CSS) variable
will only be used for changing the path for gitweb.js in gitweb.cgi like it
used to before 2.38.0 (before 564ebde3d323fc3b22534dcbb32723807771b955)

What happened instead? (Actual behavior)

The build failed with:

         GEN gitweb/gitweb.cgi
     make: *** No rule to make target 'gitweb//www/gitweb.js', needed by 
'gitweb'.  Stop.

What's different between what you expected and what actually happened?

Anything else you want to add:

This is caused by 564ebde3d323fc3b22534dcbb32723807771b955, but I am not 
sure
what exactly is the issue there, so I am only reporting this and leaving the
fix to someone else.


[System Info]
git version:
git version 2.39.2
cpu: x86_64
no commit associated with this build
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
compiler info: gnuc: 12.2
libc info: glibc: 2.36
$SHELL (typically, interactive shell): /bin/bash