Bugfix: Revised/repaired SFML2 sample

Changed the code such that the SFML 2 sample is compatible with the
revised libraries of SFML2
Bugfix: fixed error building libRocket with MinGW

When librocket was build with MinGW the wrong platform was selected. As
a consequence the wrong code was compiled which lead to corrupted
libraries.
Revert "Bugfix: fixed error building libRocket with MinGW"

This reverts commit 66b4d56779e6c5d370298d9f1ec47f56adf8003b.
Bugfix: fixed error building libRocket with MinGW

When librocket was build with MinGW the wrong platform was selected. As
a consequence the wrong code was compiled which lead to corrupted
libraries.
Revert "Bugfix: fixed error building libRocket with MinGW"

This reverts commit 773afa3a051f9907edce387f0d66142373f098cb.