gitweb: Prepare for splitting gitweb

gitweb: Prepare for splitting gitweb

Prepare gitweb for having been split into modules that are to be
installed alongside gitweb in 'lib/' subdirectory, by adding

  use lib __DIR__.'/lib';

to gitweb.perl (to main gitweb script), and preparing for putting
modules (relative path) in $(GITWEB_MODULES) in gitweb/Makefile.

This preparatory work allows to add new module to gitweb by simply
adding

  GITWEB_MODULES += <module>

to gitweb/Makefile (assuming that the module is in 'gitweb/lib/'
directory).

While at it pass GITWEBLIBDIR in addition to GITWEB_TEST_INSTALLED to
allow testing installed version of gitweb and installed version of
modules (for future tests which would check individual (sub)modules).

Using __DIR__ from Dir::Self module (not in core, that's why currently
gitweb includes excerpt of code from Dir::Self defining __DIR__) was
chosen over using FindBin-based solution (in core since perl 5.00307,
while gitweb itself requires at least perl 5.8.0) because FindBin uses
BEGIN block, which is a problem under mod_perl and other persistent
Perl environments (thought there are workarounds).

At Pavan Kumar Sankara suggestion gitweb/Makefile uses

  install [OPTION]... SOURCE... DIRECTORY

format (2nd format) with single SOURCE rather than

  install [OPTION]... SOURCE DEST

format (1st format) because of security reasons (race conditions).
Modern GNU install has `-T' / `--no-target-directory' option, but we
cannot rely that the $(INSTALL) we are using supports this option.

The install-modules target in gitweb/Makefile uses shell 'for' loop,
instead of make's $(foreach) function, to avoid possible problem with
generating a command line that exceeded the maximum argument list
length.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>