Feature: VTK lib header file with linkage macros.

[holke]

Feature request

When a library links against t8code, they cannot access T8_ENABLE_VTK.
We should make these available in t8code header.
This is possible using cmake configure_file.

Is your feature request related to a problem? Please describe.
External libraries using t8code cannot see if t8code is linked against another library (for example VTK) and with which version of that library.

Describe the solution or feature you'd like
Create a header file with

T8_ENABLE_VTK // 0 or 1 depending on whether t8code is linked against LIBNAME or not.
T8_VTK_MAJOR_VERSION // Defined only if T8_ENABLE_VTK  is true. The major version number of VTK that t8code links again.
T8_VTK_MINOR_VERSION // Defined only if T8_ENABLE_VTK  is true. The minor version number of VTK that t8code links again.  

Describe alternatives you've considered
N/A

Estimated priority

"Priority: medium" Should be solved within half a year

Additional context

[spenke91]

see #2007: We agreed it's a useful feature, but don't know whether there is a better CMake way