Unity builds were intended to speed up builds, by bundling multiple files into compilation units. However, now that ccache is available on all platforms, there is no need for unity builds anymore, as ccache stores compiled individual build objects for reuse. This change therefore removes the ability to make unity builds.
There were a few uninitialized variables in CMake files. This change will make sure we always check if a variable has been initialized before using them, or in come cases initialize them by default. This change will raise an error on CI if a developer introduced an uninitialized variable in CMake files.
Per [XLS-0095](https://xls.xrpl.org/xls/XLS-0095-rename-rippled-to-xrpld.html), we are taking steps to rename ripple(d) to xrpl(d).
This change modifies the binary name from `rippled` to `xrpld`, and creates a symlink named `rippled` that points to the `xrpld` binary.
Note that https://github.com/XRPLF/rippled/pull/5975 renamed any references to `rippled` in the CMake files and their contents, but explicitly maintained the `rippled` binary name by adding an exception. This change now undoes this exception and adds an explicit symlink instead.
This change removes unused definitions from the CMake files, moves variable definitions from `XrplSanity` to `XrplSettings` where they better belong, and updates the minimum GCC and Clang versions to match what we actually minimally support.
Per XLS-0095, we are taking steps to rename ripple(d) to xrpl(d).
This change updates the CMake files and definitions therein, plus a handful of related modifications. Specifically, the compiler files are renamed from `RippleXXX.cmake` or `RippledXXX.cmake` to `XrplXXX.cmake`, and any references to `ripple` and `rippled` (with or without capital letters) are renamed to `xrpl` and `xrpld`, respectively. The name of the binary, currently `rippled`, remains unchanged and will be updated in a separate PR. This change is purely cosmetic and does not affect the functioning of the binary.
