dnsmasq: static linking with lua doesn't need -ldl

Fixes http://autobuild.buildroot.net/results/77e/77e4123a3ddc934efedf4b09adc2436421ee70b3/



liblua only uses libdl when dynamic linking is used, and certain toolchains
(E.G.  bfin) doesn't provide a libdl - So only link against it if it is
needed.

At the same time change it pass the library in LIBS instead of LDFLAGS so it
ends up at the end of the linker cmdline.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>