Define LIBGUESTFS_HAVE_<shortname> for C API functions.

The actions each have a corresponding define, eg:

  #define LIBGUESTFS_HAVE_VGUUID 1
  extern char *guestfs_vguuid (guestfs_h *g, const char *vgname);

However functions which are for testing, debugging or deprecated do
not have the corresponding define.  Also a few functions are so
basic (eg. guestfs_create) that there is no point defining a symbol
for them.