Makefile: allow overriding libusb Makefile variables externally

Also: Add the winsock2 library to LIBS for Windows. When not
linking against it, the usage of WS2 conversion functions from
portable_endian.h would cause unresolved symbols.
Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>

Makefile

@@ -98,11 +98,13 @@ sunxi-fexc: fexc.h script.h script.c \
 	script_fex.h script_fex.c
 
 LIBUSB = libusb-1.0
-LIBUSB_CFLAGS = `pkg-config --cflags $(LIBUSB)`
-LIBUSB_LIBS = `pkg-config --libs $(LIBUSB)`
+LIBUSB_CFLAGS ?= `pkg-config --cflags $(LIBUSB)`
+LIBUSB_LIBS ?= `pkg-config --libs $(LIBUSB)`
 ifeq ($(OS),Windows_NT)
 	# Windows lacks mman.h / mmap()
 	DEFINES += -DNO_MMAP
+	# portable_endian.h relies on winsock2
+	LIBS += -lws2_32
 endif
 
 sunxi-fel: fel.c fel-to-spl-thunk.h progress.c progress.h