added missing hidapi stuff for os/2 with libusb

Makefile.os2

@@ -4,6 +4,11 @@
 # If you have GNU libiconv installed (iconv2.dll), you
 # can compile against it by specifying LIBICONV=1, e.g.:
 # wmake -f Makefile.os2 LIBICONV=1
+#
+# If you have libusb-1.0 installed (usb100.dll, libusb.h), you
+# can compile hidapi joystick support against it (experimental)
+# by specifying HIDAPI=1, e.g.:
+# wmake -f Makefile.os2 HIDAPI=1
 
 LIBNAME = SDL2
 VERSION = 2.0.20
@@ -42,6 +47,10 @@ LIBS+= $(ICONVLIB)
 !else
 LIBS+= libuls.lib libconv.lib
 !endif
+# hidapi (libusb):
+!ifeq HIDAPI 1
+CFLAGS_DLL+= -DHAVE_LIBUSB_H=1
+!endif
 # building SDL itself (for DECLSPEC):
 CFLAGS_DLL+= -DBUILD_SDL