perl: treat PERLLIB_EXTRA as an extra path again

perl: treat PERLLIB_EXTRA as an extra path again

PERLLIB_EXTRA was introduced in v1.9-rc0~88^2 (2013-11-15) as a way
for packagers to add additional directories such as the location of
Subversion's perl bindings to Git's perl path.  Since 20d2a30f
(Makefile: replace perl/Makefile.PL with simple make rules,
2012-12-10) setting that variable breaks perl-based commands instead:

 $ PATH=$HOME/opt/git/bin:$PATH
 $ make install prefix=$HOME/opt/git PERLLIB_EXTRA=anextralibdir
[...]
 $ head -2 $HOME/opt/git/libexec/git-core/git-add--interactive
 #!/usr/bin/perl
 use lib (split(/:/, $ENV{GITPERLLIB} || ":helloiamanextrainstlibdir" || "/usr/local/google/home/jrn/opt/git/share/perl5"));
 $ git add -p
 Empty compile time value given to use lib at /home/jrn/opt/git/libexec/git-core/git-add--interactive line 2.

Removing the spurious ":" at the beginning of ":$PERLLIB_EXTRA" avoids
the "Empty compile time value" error but with that tweak the problem
still remains: PERLLIB_EXTRA ends up replacing instead of
supplementing the perllibdir that would be passed to 'use lib' if
PERLLIB_EXTRA were not set.

The intent was to simplify, as the commit message to 20d2a30f
explains:

| The scripts themselves will 'use lib' the target directory, but if
| INSTLIBDIR is set it overrides it. It doesn't have to be this way,
| it could be set in addition to INSTLIBDIR, but my reading of
| [v1.9-rc0~88^2] is that this is the desired behavior.

Restore the previous code structure to make PERLLIB_EXTRA work again.

Reproducing this problem requires an invocation of "make install"
instead of running bin-wrappers/git in place, since the latter sets
the GITPERLLIB environment variable, avoiding trouble.

Reported-by: Jonathan Koren <jdkoren@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>