Replace C macros by C++ constexpr

[JanVogelsang]

In the code we make extensive use of C macros to enable/disable certain regions of code based on compiler flags. These C macros could be replaced by C++ constexpr equivalents and

#define ENABLE_FEATURE 1
#if ENABLE_FEATURE
...
#endif

by

constexpr bool enable_feature = true;
if constexpr (enable_feature) {
...
}

This way all code would always be checked by the compiler independent of the local compiler flags, which reduces possible errors that one doesn't catch locally.
Maybe there is a way for CMake to create constexpr variables directly instead of defining C macro variables.

[sameep-git]

Hi, I would like to help with this issue. I am new to open-source and nest. Could you tell me more about what your approach to this problem is? Are we trying to replace the code for every instance of this in the codebase or just some particular ones?