Variadic functions were introduced in 5.0, but the method of accessing
them changed in 5.1. This makes the code incompatible with 5.0 (it is
already incompatible since at least b2b1755), which will be removed
from the build system in a subsequent commit.

Fixes raboof#124. See also raboof#90.

Lua doesn't include an official pkg-config file, therefore we can't rely on
pkg-config variables like "V" or "major_version" - on my system, it detected
the 5.3 library/cflags and used the 5.1 lua/luac binaries (I have both installed
and there was a fallback to 5.1 if it couldn't query these variables from
pkg-config).