Add CMake option to bundle all static dependencies

[davisp]

This PR adds a new bootstrap/cmake option for *nix operating systems that allows for creating a single unified static library that contains libtiledb and all the vcpkg installed dependencies.

---

TYPE: IMPROVEMENT
DESC: Add static bundle option for static builds

[davisp]

Anyone wanting to use this needs to have armerge available on their path. If you have cargo installed you should be able to just do cargo install armerge.

[teo-tsirpanis]

For some backstory, my understanding has been that applying custom logic to transitive dependencies goes against CMake's methodology and I've been nearly convinced that it is impossible to do precisely in the general case. We used to support installing all static libraries of our dependencies alongside our own static library, which was very fragile and prone to failures when updating dependencies, and its removal in #4505 led to massive simplification of the build system.

A new feature in CMake 3.30 might change the situation, which I tried to use to no avail. If we can iterate on it and make it work I would be more open but as I commented, the proposed implementation is IMHO not general-purpose enough to make me comfortable to include in the Core's build system.

If a downstream project requires to ship a combined static library, I believe it is more appropriate to put that logic there, where the downstream project has more control over its dependencies.

[teo-tsirpanis]

Suggested change

	find_program(ARMERGE armerge)
	if (NOT ARMERGE)
	message(FATAL_ERROR "Failed to find 'armerge' for building static bundle.")
	endif()
	find_program(ARMERGE armerge REQUIRED)

Alternatively we could use libtool and lib on Windows.

[teo-tsirpanis]

Bundling all static libraries in vcpkg_installed is fragile (assumes all dependencies are being obtained via vcpkg, would need extra logic for Windows and debug libraries) and inefficient (we don't necessarily depend on all libraries in that folder).

[teo-tsirpanis]

I have noticed a recent CMake change that might help us precisely get all transitive static dependencies of a target. Here's my (unsuccessful) experiment, annotated with open questions:

# Create a static library that includes libtiledb and all of its dependencies
if (TILEDB_STATIC_BUNDLE)
    if(POLICY CMP0166)
      # TARGET_PROPERTY evaluates link properties transitively over private dependencies of static libraries.
      # Hmm, it doesn't have any effect? I'm not seeing azure-core-cpp for example in the generated Ninja file.
      cmake_policy(SET CMP0166 NEW)
    else()
      message(FATAL_ERROR "TILEDB_STATIC_BUNDLE requires CMake 3.30 or newer")
    endif()

    # Incrementally build generator expression with the library files of all of our dependencies
    set(static_deps_genex $<TARGET_PROPERTY:LINK_LIBRARIES>)
    # Expand expressions like $<TARGET_PROPERTY:tgt,INTERFACE_LINK_LIBRARIES>
    set(static_deps_genex $<GENEX_EVAL:${static_deps_genex}>)
    # Wrap each dependency target in a $<TARGET_LINKER_FILE:dep>.
    # TODO: What happens on system libraries? We might need to transform dep to
    # $<IF:$<TARGET_EXISTS:dep>,$<TARGET_LINKER_FILE:dep>,dep>. This could be done
    # with a regex replacement which I tried, but still couldn't andwer the question below.
    # The million-dollar question: How to escape the dollar?? Is it even possible?
    set(static_deps_genex $<LIST:TRANSFORM,${static_deps_genex},PREPEND,$<TARGET_LINKER_FILE:>)
    set(static_deps_genex $<LIST:TRANSFORM,${static_deps_genex},APPEND,$<ANGLE-R>>)
    # Evaluate the genex again. We go from $<TARGET_LINKER_FILE:a>;$<TARGET_LINKER_FILE:b>
    # to liba.a;libb.a
    set(static_deps_genex $<GENEX_EVAL:${static_deps_genex}>)

    # We can simplify the developer experience and post-process the existing libtiledb.a.
    # This is optional and should be done if we can resolve the concern below.
    add_custom_command(TARGET tiledb
        POST_BUILD
        # TODO: Use libtool or armerge on non-Windows.
        COMMAND lib /out:$<TARGET_FILE:tiledb> $<TARGET_FILE:tiledb> ${static_deps_genex}
        COMMAND_EXPAND_LISTS
    )

    # TODO: Clear all link_libraries of the tiledb target.
    # This sounds fragile. All our PUBLIC link_libraries are currently link-only and removing them
    # is safe, but if it stops being true, how to pick them out? There is already such target
    # in configuration_definitions, but that's undocumented and should not be exposed either way.
endif()