build: add global var for whether or not the gui is enabled

src/main.cpp

@@ -48,6 +48,7 @@ bool fReindex = false;
 bool fBenchmark = false;
 bool fTxIndex = false;
 unsigned int nCoinCacheSize = 5000;
+bool fHaveGUI = false;
 
 /** Fees smaller than this (in satoshi) are considered zero fee (for transaction creation) */
 int64 CTransaction::nMinTxFee = 10000;  // Override with -mintxfee