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.

pkgs/development/interpreters/lua-5/interpreter.nix

@@ -49,7 +49,7 @@ self = stdenv.mkDerivation rec {
   configurePhase = ''
     runHook preConfigure
 
-    makeFlagsArray+=(CFLAGS="-DLUA_USE_LINUX -O2 -fPIC${if compat then " -DLUA_COMPAT_ALL" else ""}" )
+    makeFlagsArray+=(CFLAGS='-O2 -fPIC${lib.optionalString compat " -DLUA_COMPAT_ALL"} $(SYSCFLAGS)' )
     makeFlagsArray+=(${lib.optionalString stdenv.isDarwin "CC=\"$CC\""}${lib.optionalString (stdenv.buildPlatform != stdenv.hostPlatform) " 'AR=${stdenv.hostPlatform.config}-ar rcu'"})
 
     installFlagsArray=( TO_BIN="lua luac" INSTALL_DATA='cp -d' \