[tlbuild] Fixes for LuaJIT on OpenBSD/clang

[Karl Berry]

            LIBLUAJIT_LDEXTRA="$jitlibs"

Mojca, I installed your test in Build/source/m4/kpse-luajit-flags.m4,
inside a new case for $build_os ~ openbsd*, except for the above line,
which I changed to 
   LUAJIT_LIBS="$LUAJIT_LIBS $jitlibs"

I'm never sure of anything, but as far as I can see LIBLUAJIT_LDEXTRA is
not used; KPSE_LUAJIT_DEFINES sets up LUAJIT_LDEXTRA (no LIB), but those
are flags that would normally appear at the beginning of the load line,
whereas I think we want these libs to be after the -ltexluajit.

I have no feasible way to test. Let me know if changes are necessary.
--thanks, karl.