luajit: fix host installation

luajit: fix host installation

Packages installed for the host should have their prefix set to
$(HOST_DIR)/usr, and therefore not use DESTDIR at installation time.

Using PREFIX=/usr DESTDIR=$(HOST_DIR) is wrong, and leads for example to
luajit.pc containing prefix=/usr, which means pkg-config returns
incorrect results for host-luajit.

This patch fixes the luajit package to conform to this rule.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
[Thomas: rewrite commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>