Fix warnings in CPU backend on clang
Fix linker warning on OSX

The default visibility for the NVCC files were not hidden
and that was causing the warning when building CUDA
with clang.
Fixed warning due to if/switch
Merge branch 'clang_fixes' into clang_warn

Conflicts:
	src/backend/cpu/transpose.cpp
	src/backend/cuda/transpose.cu
Merge pull request #279 from umar456/clang_warn

Fix compiler warnings on OSX