Allow user to control warnings-as-errors behaviour

[andrew-wja]

Currently, unless the user builds Caffe with -DCODE_COVERAGE=ON, the CMakefile hard-codes the -Werror flag to the compiler. This means that the build is breaking for trivial reasons.

I can understand wanting to have zero compiler warnings for releases, and I think this is a sensible default, but if I want to build with warnings, the only way is to edit the CMakefile, which is not ideal.