Add a workaround for incorrect CUDA_HOST_COMPILER setting

Recent versions of cmake set CUDA_HOST_COMPILER to CMAKE_C_COMPILER
which on OSX defaults to clang (/usr/bin/cc), but this is not a
supported cuda compiler.  This commit adds a workaround to
preemptively set CUDA_HOST_COMPILER to gcc if that compiler exists in
/usr/bin.
Removed defines in hog.h.
Added check for OpenNI in CMakeLists.txt (fixed bug 982).
Removed parameters eps and PI from hog.h (used M_PI instead of PI).
Merge pull request #32 from patmarion/0979-fix-cuda-host-compiler

Add a workaround for incorrect CUDA_HOST_COMPILER setting
Merge pull request #34 from mmunaro/master

Removed defines from hog.h.
Added check for OpenNI in CMakeLists.txt (fixed bug 982).
Merge pull request #35 from mmunaro/master

Added check for OpenNI in CMakeLists.txt (fixed bug 982).