sysdeps: support regex in altdeps
This adds support for using regex in altdeps by adding a new attribute 'regex' to the deps and altdeps entries.
We also add /usr/lib to the searched directories. This change is needed because clang installs its headers into /usr/lib/llvm-/include/clang rather than putting them into /usr/include/llvm-.
When resolving altdeps we now check if an altdep is specified via a regex and then try find a matching header in the list of search include directories.