Fix for issue 3528: cmake generator expression space removed

[edwinsolisf]

Fixes CMake Generator Expression Issue with a system with no OpenCL installed compiles arrayfire without the OpenCL backend

Description

• Is this a new feature or a bug fix?: Bug Fix
• Why these changes are necessary: Fixes CMake Build
• Potential impact on specific hardware, software or backends: Allows building non OpenCL ArrayFire backends without OpenCL installed in Linux
• New functions and their functionality: None
• Can this PR be backported to older versions?: Yes
• Future changes not implemented in this PR: None

Fixes: #3528

Changes to Users

• No options added to the build
• No action required by the user

Checklist

• Rebased on latest master
• Code compiles
• Tests pass
• Functions added to unified API
• Functions documented

[HugoPhibbs]

Is it also necessary to remove the space in the cmake expressions on lines 46-47?

    $<$<BOOL:${OpenCL_FOUND}>:${CMAKE_CURRENT_SOURCE_DIR}/opencl.cpp>
    $<$<BOOL:${CUDA_FOUND}>:${CMAKE_CURRENT_SOURCE_DIR}/cuda.cpp>

[edwinsolisf]

@HugoPhibbs It's not necessary. The problem reported was caused due to cmake generator expressions behaving weirdly with whitespace. See documentation. But I'll update those lines too, for consistency.