While the configure script sets the EXPATDIR environment variable to
whatever value was passed to its option --with-expat as the prefix of
the location of the expat library and headers, the Makefile ignored it.
This patch fixes this bug.
Signed-off-by: NSerge van den Boom <svdb@stack.nl>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>