Fixed linker error running wlc check in check_sawsim.

Fixed linker error running wlc check in check_sawsim.

Obviously, the wlc check should be in check_tension_model, but that
doesn't exist yet.  When it does, it will still need the wlc()
declaration, since wlc() is not exported directly by tension_model.h.
I also added protective #ifndef-#def-#endif protection to the headers.

TODO: Determine which modules depend on which other modules and make
the appropriate includes in the headers themselves.  Then the .c files
need only include the headers they use directly.