Resolve successive config mismatched NGenConfig.h

[program--]

This PR resolves #681 by outputting the configured NGenConfig.h to the specified build directory instead of the source include directory.

i.e., in the example given in #681, now we will have 2 NGenConfig.h headers that correctly represent the build directory's configuration:

$ cmake -B build_dir_A -S . -DNGEN_WITH_MPI=OFF -DNGEN_WITH_PYTHON=OFF -DNGEN_WITH_TESTS=OFF
$ cmake -B build_dir_B -S . -DNGEN_WITH_MPI=ON -DNGEN_WITH_PYTHON=OFF -DNGEN_WITH_TESTS=OFF
$ diff build_dir_A/include/NGenConfig.h build_dir_B/include/NGenConfig.h

12c12
< #define NGEN_WITH_MPI 0
---
> #define NGEN_WITH_MPI 1
54c54
<     NGEN_WITH_MPI: OFF
---
>     NGEN_WITH_MPI: ON
67a68,75
>   MPI (C):
>     Version: 4.0
>     Library: /usr/local/lib/libmpi.so
>     Include: /usr/local/include
>   MPI (CXX):
>     Version: 4.0
>     Library: /usr/local/lib/libmpicxx.so,/usr/local/lib/libmpi.so
>     Include: /usr/local/include

Additions

• New target for the configuration header given a build, NGen::config_header that includes ${build_directory}/include.
• Configures the Python build version source file in src/core to ${CMAKE_CURRENT_BINARY_DIR}/src/core/NGen_Python_Build_Versions.cpp and adds it as a source to NGen::core.

Testing

Using the example above, built target ngen and target all successfully for both build configurations.

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• Reviewers requested with the Reviewers tool ➡️

[PhilMiller]

This may not be the perfect way to do things as CMake goes, but I don't really know the right way, and I'm pretty sure it will resolve the problem for now.