Add support for adding link dependencies via --cxx-link.

[bbannier]

We already supported specifying additional include directories, but
injecting additional link dependencies into the link always required
manually compiling and linking files. So while users could already
declare e.g., existing functions with &cxxname, we provided no tooling
to use such functions if we were defined in external libraries or static
archives.

This patch adds support for that. One can now reference static archives
or shared libraries to include in the link by passing them with
--cxx-link <lib>. This will copy symbols for static archives or
introduce a shared library dependency into the created HLTO file and
needs only to be specified for the step creating the HLTO file.

Closes #1236.

[bbannier]

@rsmmr, I still need to figure out how to make the test case work on freebsd13, but I think the idea for the user API is where I want it to be. Could you take a look?

[rsmmr]

I think my main question is if an environment variable is the right mechanism here. On the plus side, it matches what we do for include paths. On the other hand, it seems unusual to pass specific libraries/targets through an environment variables (whereas it's common for a search path like the include dirs). The alternative would be a command line option (and we could add one for include directories, too, then on top of the env var).

[bbannier]

I think my main question is if an environment variable is the right mechanism here. On the plus side, it matches what we do for include paths. On the other hand, it seems unusual to pass specific libraries/targets through an environment variables (whereas it's common for a search path like the include dirs). The alternative would be a command line option (and we could add one for include directories, too, then on top of the env var).

I think we want to support cases where Spicy analyzers use e.g., the CMake macro spicy_add_analyzer which does not invoke spicyz directly (this is e.g., used in the package template). If we add a command line argument for libs we sjould add a new LIBS arg to that macro as well. Let me check on that.

[bbannier]

Updated this as discussed offline. Libs can now be passed with the flag --cxx-link (potentially used multiple times).

I also tried adding the global way to pass arbitrary command line args to our drivers that you mentioned (e.g., HILTI_OPTIONS="-c --cxx-link foo.so" would set the two command line options). This will be rather tricky to implement and use since we somehow would need to reconstruct int argc and char** argv from that which becomes hard when arguments need escaping e.g., due to embedded whitespace. I'd suggest we do not attempt this, but instead add a new named argument to spicy_add_analyzer over in zeek/spicy-plugin as a follow-up.