t/Makefile: Can't include GIT-BUILD-OPTIONS, it's a .sh

t/Makefile: Can't include GIT-BUILD-OPTIONS, it's a .sh

Change the smoke testing portion of t/Makefile not to include
GIT-BUILD-OPTIONS. It's a shellscript, not a Makefile snippet, so it
had the nasty side-effect of sneaking e.g. SHELL_PATH = '/bin/sh'
(with quotes) everywhere.

Just add our own PERL_PATH variable as a workaround. The t/Makefile
already has e.g. an equivalent SHELL_PATH and TAR option which
duplicate the definitions in GIT-BUILD-OPTIONS.

Reported-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>