lua: fix linking under glibc

Compilation of liblua itself works, but when other packages link against
it, the linker starts throwing undefined references to a bunch of math
functions in libm.

First discovered in a failed attempt to transition a package to uClibc++.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
[fix commit title capitalization]
Signed-off-by: David Bauer <mail@david-bauer.net>