[CMake] Fix #631

This fixes a bug in the bitcode build system where the build would
fail if the build directory was a symbolic link (i.e. create a symbolic
link for the root of the build tree and try to do the build in that
directory).

The problem was that `DIR_SUFFIX` implicitly assumed that there was
only one way to refer to the build tree which is an incorrect assumption
in the presence of symbolic links. This has been fixed by using the
`$(realpath)` GNU make built in to resolve all symbolic links.

An additional sanity check has been added to check that `SRC_DIR`
exists.

0 files changed, 0 insertions, 0 deletions