BUILD: makefile: do not restrict Lua's prepend path to empty LUA_LIB_NAME

BUILD: makefile: do not restrict Lua's prepend path to empty LUA_LIB_NAME

The HLUA_PREPEND_PATH and HLUA_PREPEND_CPATH settings were only applied
when LUA_LIB_NAME was empty, otherwise they were silently ignored. Let's
take them out of that conditional block as this makes no sense to enforce
such a restriction (the main reason in fact is that this whole block is
unreadable).

Also take this opportunity to unfold the last two imbricated tests of
LUA_LIB_NAME and put comments around certain blocks to know what "endif"
matches what "if".