Improved Makefile

Use sdl-config to get SDL compile and link flags.
Made it possible to compile for non-MIPS archs by overriding only CXX.
Gather list of sources using wildcard.
Compile with warnings enabled also in non-debug mode.
Strip using GCC's -s option.
Put output in a different directory depending on arch and debug mode.
Do not omit frame pointer in debug mode; it is useful for debugging.
Removed .elf extension for executable.