fix bundled luajit build

[wheybags]

When I attempt to build the current git master on linux, I get a link error:

/usr/bin/ld: /tmp/pip-req-build-qmhpkr3b/third-party/luajit20/src/libluajit.a(lj_err.o): relocation R_X86_64_TPOFF32 against `static_uex' can not be used when making a shared object; recompile with -fPIC

Command line used:

pip install -v --no-clean git+https://github.com/scoder/lupa.git@6e3b2fcdaacfda7c1e43eb3c5dfc97c96906b00a --global-option="--use-bundle"

So I added -fPIC to the CFLAGS variable when building the bundled luajit.