lua: don't always set LUA_USE_LINUX
lua: don't always set LUA_USE_LINUX

This is supposed to be automatically set by Lua's Makefile if PLAT is
set appropriately, but it was being overridden by us overridding
CFLAGS.  Setting it manually was a hack.  The correct thing to do was
to make sure SYSCFLAGS (where Lua's Makefile puts LUA_USE_LINUX) was
still included in our custom CFLAGS.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs