build: use -mwindows for gui targets when linking with mingw

src/m4/bitcoin_qt.m4

@@ -109,6 +109,10 @@ AC_DEFUN([BITCOIN_QT_CONFIGURE],[
        AX_CHECK_LINK_FLAG([[$base_frameworks]],[QT_LIBS="$QT_LIBS $base_frameworks"],[AC_MSG_ERROR(could not find base frameworks)])
      ])
     ;;
+    *mingw*)
+       BITCOIN_QT_CHECK([
+         AX_CHECK_LINK_FLAG([[-mwindows]],[QT_LDFLAGS="$QT_LDFLAGS -mwindows"],[AC_MSG_WARN(-mwindows linker support not detected)])
+       ])
   esac
 
 
@@ -137,6 +141,7 @@ AC_DEFUN([BITCOIN_QT_CONFIGURE],[
 
   AC_SUBST(QT_INCLUDES)
   AC_SUBST(QT_LIBS)
+  AC_SUBST(QT_LDFLAGS)
   AC_SUBST(QT_DBUS_INCLUDES)
   AC_SUBST(QT_DBUS_LIBS)
   AC_SUBST(QT_TEST_INCLUDES)

src/qt/Makefile.am

@@ -341,6 +341,7 @@ bitcoin_qt_LDADD += $(LIBBITCOIN_WALLET)
 endif
 bitcoin_qt_LDADD += $(LIBBITCOIN_CLI) $(LIBBITCOIN_COMMON) $(LIBLEVELDB) $(LIBMEMENV) \
   $(BOOST_LIBS) $(QT_LIBS) $(QT_DBUS_LIBS) $(QR_LIBS) $(PROTOBUF_LIBS) $(BDB_LIBS)
+bitcoin_qt_LDFLAGS = $(QT_LDFLAGS)
 
 # forms/foo.h -> forms/ui_foo.h
 QT_FORMS_H=$(join $(dir $(QT_FORMS_UI)),$(addprefix ui_, $(notdir $(QT_FORMS_UI:.ui=.h))))

src/qt/test/Makefile.am

@@ -41,5 +41,6 @@ endif
 test_bitcoin_qt_LDADD += $(LIBBITCOIN_CLI) $(LIBBITCOIN_COMMON) $(LIBLEVELDB) \
   $(LIBMEMENV) $(BOOST_LIBS) $(QT_DBUS_LIBS) $(QT_TEST_LIBS) $(QT_LIBS) \
   $(QR_LIBS) $(PROTOBUF_LIBS) $(BDB_LIBS)
+test_bitcoin_qt_LDFLAGS = $(QT_LDFLAGS)
 
 CLEANFILES = $(BUILT_SOURCES) *.gcda *.gcno