Fix PE header of the VS2015 builds so they run on Windows XP.

Fix PE header of the VS2015 builds so they run on Windows XP.

By default the VS2015 linker produces binaries with the minimum
version fields in the PE header set to 06.00, which causes XP not to
recognise them as valid binaries at all. But there's no other reason
VS2015-built binaries _can't_ run on versions of Windows as old as XP;
so here I add the link option to set those fields to 05.01 which makes
XP like them again.

This only applies to the 32-bit build, because the VS2015 64-bit
linker refuses to lower the min version field to under 06.00.