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>