[CMake] add_definitions: if option contains space and using FindCUDA, it won't get added to compiler call

[xyzdragon at fastmail.fm]

Here is a minimal working-example without FindCUDA:

main.cpp:

    int main(void){ return 0; }

CMakeLists.txt:

    cmake_minimum_required(VERSION 3.5)
    add_definitions( "-D USE_FEATURE" )
    add_library( mimi main.cpp )
    target_link_libraries( mimi )

the output will be:

    /usr/bin/g++-4.9 -D USE_FEATURE -o CMakeFiles/mimi.dir/main.cpp.o -c
    ~/cmakeBug/main.cpp

Meaning the option has been correctly added. Note that the space after
`-D` is allowed and completely fine.

Now the CMakeLists.txt with FindCUDA:

    cmake_minimum_required(VERSION 3.5)
    find_package( CUDA )
    add_definitions( "-D USE_FEATURE" )
    add_definitions( "-DUSE_FEATURE2" )
    SET_SOURCE_FILES_PROPERTIES( main.cpp PROPERTIES
    CUDA_SOURCE_PROPERTY_FORMAT OBJ )
    cuda_add_library( mimi main.cpp )
    target_link_libraries( mimi )

The compile command produced now is:

    /usr/bin/nvcc ~/cmakeBug/main.cpp -x=cu -c -o
    ~/cmakeBug/build/CMakeFiles/mimi.dir//./mimi_generated_main.cpp.o
    -ccbin /usr/bin/gcc-4.9 -m64 -DUSE_FEATURE2 -Xcompiler ,\"-g\"
    -DNVCC -I/usr/include -I/usr/include

Meaning `-D USE_FEATURE` was silently ignored (resulting in me wondering
for many hours why it just won't work), but `-DUSE_FEATURE2` has been
added correctly.

Note that I also had a similar problem when trying to (admittedly rather
clumsily) adding multiple options:

    add_definitions( "-DUSE_FEATURE ${FFTW_DEFINITIONS}" )
    add_definitions( "-DUSE_FEATURE" ${FFTW_DEFINITIONS} )

The first version will be completely ignored, but the second version
works fine. I highly think that this is because of the space in the
middle.

I found this bug, because I want to do `add_definitions( "-isystem
/user_system_library" )` in order to implement a workaround for this
bug: https://cmake.org/Bug/view.php?id=14415

Another problem is, that findCUDA ignores all definitions which don't
begin with `-D` I think. at least `add_definition("-isystem/mimi")` will
be ignored. This means I can't test, what happens with paths containing
spaces.

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