Added --static and --clang flags to SConstruct.

- Added --static flag to SConstruct, which forces static linking
  (-static linkflag) on.

- Added --clang flag to SConstruct, which forces use of Clang for C/C++
  compilation, because at this point Clang's diagnostics are still much
  better than GCC 4.7's (if for no other reason than coloring).

- Build with EIKI_GDB_IF_TTY for debug and checked builds, which drops
  into GDB when a fatal signal is caught and stdin is a tty.