compiler.pri: explicitly target Windows XP when specifying output binary's subsystem.

MSVS 2012 and 2013 default to outputting binaries that require Vista or greater.
compiler.pri: explicitly specify -ARCH:SSE for MSVS 2012 and 2013.

The C++ compiler in these versions os Visual Studio default to
generating SSE2 code, unless this is overridden with /ARCH.