Fix build of console and GUI executables for Windows

It was broken by commit 8ecc89f6 which
moved the SDL linker flags from macro libs_softmmu to macro SDL_LIBS.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Message-id: 20171116163732.31584-1-sw@weilnetz.de
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

Makefile.target

@@ -22,7 +22,7 @@ QEMU_PROG_BUILD = $(QEMU_PROG)
 else
 # system emulator name
 QEMU_PROG=qemu-system-$(TARGET_NAME)$(EXESUF)
-ifneq (,$(findstring -mwindows,$(libs_softmmu)))
+ifneq (,$(findstring -mwindows,$(SDL_LIBS)))
 # Terminate program name with a 'w' because the linker builds a windows executable.
 QEMU_PROGW=qemu-system-$(TARGET_NAME)w$(EXESUF)
 $(QEMU_PROG): $(QEMU_PROGW)