Scale2x v4.0

• Added a new SSE2 implementation using intrinsics that replaces the old
  MMX one [Thomas Bernard].
  To enable it, you must build with the -msse2 option.
• Made source ANSI C compliant [Thomas Bernard].
• Windows binaries built with gcc 4.9.3 using the MXE cross compiler at
  commit 62bcdbee56e87c81f1faa105b8777a5879d4e2e with targets
  i686-w64-mingw32 and x86_64-w64-mingw32.

Scale2x v3.2

• Revised the license. Now it's only GPL2+.
• Windows binary built with gcc 4.8.1 using the MXE cross compiler 2.23,
  with targets i686-w64-mingw32. Before the target was i686-pc-mingw32.