Automatically merge target libraries

After 2.5.8, we can enable automatic merging of all dependent static libraries by setting the build.merge_archive strategy, for example:

add_rules("mode.debug", "mode.release")

target("add")
    set_kind("static")
    add_files("src/add.c")
    add_files("src/subdir/add.c")

target("sub")
    set_kind("static")
    add_files("src/sub.c")
    add_files("src/subdir/sub.c")

target("mul")
    set_kind("static")
    add_deps("add", "sub")
    add_files("src/mul.c")
    set_policy("build.merge_archive", true)

The mul static library automatically merges the add and sub static libraries to generate a complete libmul.a library containing add/sub code.

This merge is relatively stable and complete, supports ar and msvc/lib.exe, also supports the merge of static libraries generated by the cross-compilation tool chain, and also supports static libraries with the same name obj file.

Merge specified static library files

If the automatic merge does not meet the requirements, we can also actively call the utils.archive.merge_archive module to merge the specified static library list in the after_link stage.

target("test")
    after_link(function (target)
        import("utils.archive.merge_staticlib")
        merge_staticlib(target, "libout.a", {"libfoo.a", "libbar.a"})
    end)

Use add_files to merge static libraries

In fact, our previous version already supports merging static libraries through add_files("*.a").

target("test")
    set_kind("binary")
    add_files("*.a")
    add_files("*.c")

Related issues: #1638