Copy rather than symlink autoconf files
Linux compilation fixes

- Restored missing #include <signal.h> incorrectly removed in 6f462a9e8c.
  Removed processor-specific macro; the raise(SIGTRAP) approach is used on all
  Linux archs.
- Added missing GLU_LIBS to radiant module (these were probably included by one
  of the GTK macros previously).
PrefabSelector compiles on Linux now