The compiling rewriter fails in the nightly builds due to referring to a non existing sysroot

[mlaveaux]

In the mcrl2compilerewriter script the compiler -isysroot flag is hardcoded to a specific macOS SDK version, in this case 14.2, based on CMAKE_OSX_SYSROOT. However, such a version does not exist on certain macOS versions. In particular it does not even exist on the build server that generates the nightly package. The reason for this is that we set DCMAKE_OSX_DEPLOYMENT_TARGET to an older target to make sure that the executables can be used on older macbooks, but this breaks the compiling rewriter. It is not clear how to resolve this issue properly.