- Select extension library directories by globbing extconf.rb.
- Exclude extensions for test before uniquifing bundled gems.
- Use `Dir.glob` base option to remove prefixed srcdir.

[Bug #21818]

Currently exceptions can be sent across ractors,
but because of a limitation in the TypedData API,
the exception backtrace is duped as an empty backtrace.

The problem is that backtraces are embedded objects,
hence the classic `rb_class_alloc(klass)` API is insufficient
because we need to know the size of the Backtrace object we're
duping to instantiate the copy.

This is worked around by changing Ractors to call `#clone` on objects
rather than use `rb_obj_clone`, and to implement `Thread::Backtrace#clone`
to properly clone the variable size object.

Build and target executable suffixes are unrelated in
cross-compilation.

This reverts commit d256629bf9e194838d1837be74dcc0b8ff0bcfd6.

When building on macOS, we get a warning about duplicate libraries

    ld: warning: ignoring duplicate libraries: '-ldl', '-lobjc', '-lpthread'

To fix it, we now append $(MAINLIBS) to LIBRUBYARG_SHARED (when shared
is enabled), matching what's already done for LIBRUBYARG_STATIC. And we
remove the now-redundant $(MAINLIBS) from the $(PROGRAM) link line,
since $(LIBRUBYARG) carries it in both cases.

A workaround for GH-12563#issuecomment-3691590489.