BUILD: fix automatic inclusion of libdl.

Last commit ecc9547 ("BUILD: lua: it miss the '-ldl' directive") broke
build on systems without libdl (eg: FreeBSD). Since lua requires libdl
on some systems, let's simplify this by adding a USE_DL build directive
to enable/disable use of libdl. It's set by default on all linux flavors.
1 file changed