The CI job for Mac seems to have updated the image and it's failing to
build when it used to work before. The way that LLVM is installed now
has changed.

This commit fixes the paths of LLVM-related binaries and headers to
point to their new locations.

The CI job for Mac seems to have updated the image and it's failing to
build when it used to work before. The permissions of top-level
directories in the image now prevent us from installing ikos at the top
level.

This change makes the CI job install IKOS in the user's HOME directory
instead.

This commit adds a missing include that leads to an error in newer
versions of clang.

IKOS currently considers that the first argument of snprintf should
never be null. However, a first argument null is allowed when the second
argument is zero.

This commit introduces a check for that second value; only when it is
not the literal zero is the first argument checked. The check applies to
the null dereference checker, the buffer overflow checker and the
numerical execution engine.

This check is not perfect: the second argument could also be a variable
with the value or function call that returns zero. However, since
passing the literal zero is the most common scenario, we check for that
case for now.

The badges at the top of the README file refer to a specific release (tag).

This commit updates the badges to point to the upcoming 3.4 release.