perl/Makefile: Unset INSTALL_BASE when making perl.mak

perl/Makefile: Unset INSTALL_BASE when making perl.mak

PREFIX and INSTALL_BASE are mutually exclusive. If both are supplied
by INSTALL_BASE being set in PERL_MM_OPT ExtUtils::MakeMaker will
produce an error:

    $ echo $PERL_MM_OPT
    INSTALL_BASE=/home/avar/perl5
    $ make -C perl  PERL_PATH='/usr/bin/perl' prefix='/home/avar' perl.mak
    make: Entering directory `/home/avar/g/git/perl'
    /usr/bin/perl Makefile.PL PREFIX='/home/avar'
    Only one of PREFIX or INSTALL_BASE can be given.  Not both.
    make: *** [perl.mak] Error 255
    make: Leaving directory `/home/avar/g/git/perl'

Change the perl Makefile to work around this by explicitly unsetting
INSTALL_BASE.

INSTALL_BASE is set in PERL_MM_OPT by e.g. the popular local::lib
package, from its documentation:

    eval $(perl -I$HOME/perl5/lib/perl5 -Mlocal::lib 2>/dev/null)

Many other environments might also have set PERL_MM_OPT before
building Git. This change enables us to build in these environments.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>