Serial backend portability

[plavin]

Clang doesn't support #pragma novector. Intel does, and I'm not sure about gcc. We need to support pragmas to disable auto-vectorization for these three compilers.

The file in question is src/serial/serial-kernels.c.

[avose]

For Clang, one might consider:
#pragma clang loop vectorize(disable)

For GCC, you could probably turn off auto vectorization for the whole file with a compile-time flag if that's acceptable:
-fno-tree-vectorize (after -O3 on the command line).

[plavin]

Thanks, that saves me a lot of reading. (It's nice to see you're still checking in on Spatter!)