CMake: create a compile_commands.json symlink

CMake: create a compile_commands.json symlink

Modern IDEs such as VSCode use a separate build directory for
configuring and compiling a project (such as our example steps).

Unfortunately, this sometimes confuses language servers such as clangd
that might fail to find the correct compile_commands.json in that build
directory. Let's work around this issue by simply creating a symlink
from the source directory pointing to the compile_commands.json file.

As a sanity check, if there is already a compile_commands.json file
present, or if the source and build directory are the same we simply do
nothing.