Fix Makefile path for Lua 5.1.5

The Lua GitHub repository doesn't have the 5.1.5 tag, and the code
downloaded from https://www.lua.org/ftp has different directory layout.

setup.py

@@ -263,6 +263,9 @@ def use_bundled_lua(path, macros):
 
     # Parse .o files from 'makefile'
     makefile = os.path.join(path, "makefile")
+    if libname == 'lua51':
+        # Lua 5.1.5 source code is downloaded from lua.org/ftp
+        makefile = os.path.join(path, "src", "Makefile")
     match_var = re.compile(r"(CORE|AUX|LIB|ALL)_O\s*=(.*)").match
     is_indented = re.compile(r"\s+").match
 
@@ -300,7 +303,7 @@ def use_bundled_lua(path, macros):
         os.path.splitext(obj_file)[0] + '.c' if obj_file != 'lj_vm.o' else 'lj_vm.s'
         for obj_file in obj_files
     ]
-    if 'lua52' in path:
+    if libname == 'lua52':
         lua_sources.extend(['lbitlib.c', 'lcorolib.c', 'lctype.c'])
     src_dir = os.path.dirname(makefile)
     ext_libraries = [