package/luajit: build host variant dynamically

package/luajit: build host variant dynamically

While building libevas.so for the host-efl with luajit support, the link fail
when libluajit is build statically:

/usr/bin/ld: output/host/usr/lib/libluajit-5.1.a(ljamalg.o): relocation R_X86_64_32S against `.rodata' can not be used when making a shared object; recompile with -fPIC
output/host/usr/lib/libluajit-5.1.a: error adding symbols: Bad value
collect2: error: ld returned 1 exit status

Build luajit dynamically in order to build host-efl with luajit support.

Also, passing HOST_LDFLAGS is needed so that a proper rpath is added
to the resulting binaries. Note that it is expected to have
HOST_LDFLAGS assigned to TARGET_LDFLAGS, it's just that luajit has a
somewhat non-conventional naming of variables. This is needed to avoid
the following error from the rpath sanity check:

***
*** ERROR: package host-luajit installs executables without proper RPATH:
***   output/host/usr/bin/luajit

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>