BUILD: makefile: make sure LUA_INC and LUA_LIB are always initialized

BUILD: makefile: make sure LUA_INC and LUA_LIB are always initialized

While LUA_INC is sometimes set in the makefile (only when LUA_LIB_NAME
is not set), LUA_LIB is never pre-initialized and faces the risk of
being accidently inherited from the environment. Let's make sure both
are properly reset first when not explicitly set. For this we always
set LUA_INC based on the autodetection if it's not set, and always
pre-initialize LUA_LIB to empty. This also helps make that block
slightly less difficult to understand.