Drop GenerateExportHeader and WriteCompilerDetectionHeader

`GenerateExportHeader` doesn't do anything special that can't already be known before building the project (i.e. a regular, static header file has no disadvantages over it), and it creates issues in [MultiArch](https://wiki.debian.org/Multiarch) environments, as the generated export header is compiler dependant (related to #35). See https://github.com/zyantific/zycore-c/pull/41 for an example.

`WriteCompilerDetectionHeader` already generates issues in cmake-init, as it isn't available in CMake versions older than 3.2; at the same time, the module got [removed in CMake 3.20](https://cmake.org/cmake/help/latest/policy/CMP0120.html).