Fix CaDiCaL auto-download on macOS (#6828)

Fix CaDiCaL auto-download on macOS (#6828)

* Fix CaDiCaL auto-download on macOS

If we are auto-downloading CaDiCaL, we are manually instantiating its makefile.
To do that, we use `CMAKE_CXX_COMPILER` for the compiler and assemble some
flags. However, we are missing the platform dependent flags. Specifically, we
need to set `-isysroot` on macOS to make sure that the header files are found
because they are not at /usr/include on newer versions of Apple's XCode [0].
Unfortunately, I could not find a CMake variable with the platform specific
flags. They are assembled here [1]. To solve this problem, the commit checks if
`CMAKE_OSX_SYSROOT` is set and adds a corresponding compiler flag if it is.

[0] https://developer.apple.com/documentation/xcode-release-notes/xcode-10-release-notes
[1] https://github.com/Kitware/CMake/blob/d60d6c269ae7ad15adbb82028e9ab50290db2a2b/Source/cmLocalGenerator.cxx#L1900-L1923