Windows XP vs Visual Studio 2012

I released a new version (2.4.1) of the x86 builds of PngOptimizer. After I switched to Visual Studio 2012 I fell into a trap: it was generating an incompatible executable for Windows XP.

Hopefully a solution exists so I used it for the 32 bits versions of PngOptimizer and PngOptimizerCL.

I left unchanged the x64 builds, including Linux, so they are still tagged 2.4. The source archive was updated though.