Add --disable-wallet option to build system

Make it possible to build Bitcoin without wallet
(and thus without BDB) so that it only functions as node.

src/qt/Makefile.am

@@ -197,7 +197,7 @@ endif
 bitcoin_qt_CPPFLAGS = $(AM_CPPFLAGS) $(QT_INCLUDES) \
   -I$(top_srcdir)/src/qt/forms
 bitcoin_qt_SOURCES = bitcoin.cpp
-bitcoin_qt_LDADD = libbitcoinqt.a $(LIBBITCOIN_SERVER) $(LIBBITCOIN_CLI) $(LIBBITCOIN_COMMON) $(LIBLEVELDB) $(LIBMEMENV) \
+bitcoin_qt_LDADD = libbitcoinqt.a $(LIBBITCOIN_SERVER) $(LIBBITCOIN_WALLET) $(LIBBITCOIN_CLI) $(LIBBITCOIN_COMMON) $(LIBLEVELDB) $(LIBMEMENV) \
   $(BOOST_LIBS) $(QT_LIBS) $(QT_DBUS_LIBS) $(QR_LIBS) $(PROTOBUF_LIBS) $(BDB_LIBS)
 
 # forms/foo.h -> forms/ui_foo.h