PSW build failed with DEBUG=1 option

When I try to compile the PSW with "DEBUG=1", an error occurs.

Here is the building log,

```
make[7]: Entering directory '/home/dajiejie/linux-sgx/external/dcap_source/QuoteGeneration/quote_wrapper/quote/linux'
../qe_logic.cpp: In function ‘void* get_qpl_handle()’:
../qe_logic.cpp:259:13: error: ‘old_libname_used’ was not declared in this scope
old_libname_used = true;
^~~~~~~~~~~~~~~~
Makefile:87: recipe for target '../qe_logic.o' failed
make[7]: *** [../qe_logic.o] Error 1
```

FYI, this error won't happen when I just do `make psw`.