Added an app that uses the C backend to make sure it doesn't break.

It only covers a very simple use of the C backend, but it can be
expanded as we find bugs.
More portable cpuid code.
Clarification in INSTALL
Manually include sse2 for the blur reference code.
Slight cleanup of simd_op_check
Made the code still compile if asserts are off.

Turning off asserts requires messing with Util.h, but I thought it should at
least be possible, e.g. to test the performance implications of the asserts.