Add find_package and add_subdirectory CMake support.

[alliepiper]

To validate installation + packages, run the new tests:

mkdir build
cd build
cmake -Dlibcudacxx_ENABLE_CMAKE_TESTS=ON ..
ctest --output-on-failure

Tests are failing until #241 is merged.

[alliepiper]

Last push fixed a bug in the versions checks (cc: @robertmaynard)

[wmaxey]

VDVS: http://scdvs.nvidia.com/Build_Results?virtualId=1000081020&which_page=current_build
VDVS: http://ausdvs.nvidia.com/Build_Results?virtualId=1001080494&which_page=current_build

[alliepiper]

@wmaxey This should be ready to go -- I've been using the package in thrust successfully in NVIDIA/thrust#1605 👍

[alliepiper]

BTW @wmaxey -- you'll need to ping the builds folks to make sure that these three files get packaged into CTK. See NVBug 3273504 where we did this for Thrust/CUB.

$SRC_DIR/lib/cmake/libcudacxx/libcudacxx-config.cmake         -> $CTK_ROOT/lib/cmake/libcudacxx/libcudacxx-config.cmake
$SRC_DIR/lib/cmake/libcudacxx/libcudacxx-config-version.cmake -> $CTK_ROOT/lib/cmake/libcudacxx/libcudacxx-config-version.cmake
$SRC_DIR/lib/cmake/libcudacxx/libcudacxx-header-search.cmake  -> $CTK_ROOT/lib/cmake/libcudacxx/libcudacxx-header-search.cmake