Fix various msvc compiler warnings. With msvc the compiler flags are explicit passed to nvcc with -Xcompiler.

According to the cuda documenation (https://docs.nvidia.com/cuda/cuda-math-api/group__CUDA__MATH__DOUBLE.html#group__CUDA__MATH__DOUBLE_1g6d36757715384dc18e0483aa1f04f6c7)
pow() gets two double arguments. With the newest VisualStudio Version 16.8 these arguments needs to be passed as double explicitly.

Remove VirtualFluidsDefinitions.h header file.
Remove redundant compiler definitions from gpu.cmake.

Make the cmake project more flexible while removing the CMAKE_SOURCE_DIR usage.

Make the cmake project more flexible while removing the CMAKE_SOURCE_DIR usage.

Remove unused stuff and the global include from VirtualFluidsDefinition.h. Adds missing export includes.

Warning: [-Wbackslash-newline-escape]
https://stackoverflow.com/questions/31673945/clang-backslash-and-newline-separated-by-space-in-single-line-comment