D3D: Don't pass D3D10_SHADER_WARNINGS_ARE_ERRORS to the compiler in debug builds.

For quite a while this has been causing integer division to generate a warning as error, blocking shader compiling. This means probably no one has even been running D3D in debug builds...
I tried disabling the warning with a #pragma, but it doesn't seem to apply when this flag is used.
Merge pull request #359 from shuffle2/d3d-disable-warnings-as-errors

D3D: Don't pass D3D10_SHADER_WARNINGS_ARE_ERRORS to the compiler in debug builds.