Seperate glX from HAVE_OPENGL, for platforms that have both an X server and a more official way to do OpenGL, explicitly check for glX on Mac OS X, and use SDL_LoadObject for platforms that have glX but don't have dlopen().

## Makefile.am for the RISC OS joystick driver for SDL

noinst_LTLIBRARIES = libjoystick_riscos.la
libjoystick_riscos_la_SOURCES = $(SRCS)

# The SDL joystick driver sources
SRCS =  SDL_sysjoystick.c