Document specifying compiler path when using CMake

[karkhaz]

This commit adds a note to COMPILING.md, explaining that the full path
to the compiler needs to be given to CMake in order to build on macOS.
This is a workaround for the issue in #4956

[smowton]

Better than nothing, but can we have cmake make a configure-time check and emit a helpful error if it notices we're on OSX and err.h isn't found?

[karkhaz]

I did try this back when I reported the bug, but it didn't work reliably. The problem isn't that err.h doesn't exist, it's that the clang that CMake uses (sometimes) doesn't have err.h in the include path. So you would either need to run clang on a test program at configure-time (I didn't try doing that, not sure whether it's possible or idiomatic), or you would need to detect the macOS minor version number.

I was focusing on trying to fix the bug (e.g. if the minor version is >14, then override CMAKE_C_COMPILER), but that's also bad if the user wants to set a different compiler entirely. You seem to be suggesting to let it fail but to emit a warning, which could work...as long as we can reliably detect when the build would actually fail.

So in summary: I'm not sure what the fix is, it isn't as obvious as it might seem. But I'd like to document the workaround meanwhile. I can't count the number of times that I've had to point people to #4956 when they've tried to compile CBMC by following the instructions, we can remove this from the documentation when it's actually fixed.