`llvm-strip-7` is a sane valid strip command that LLVM 7 ships, albeit
it does not understand `--version`.  It is a bad idea to check that
option.  Instead just see if the command actually strips something.  A
copy of `/bin/sh` should suffice.  That file must be ubiquitous.

In case of cross-compilation, GCC might provide its own linker.  Its
behaviour seems slightly different from that of gcc(1).  This is not a
big deal for normal situations, but the difference can cause libtool to
go mad.

We ship bundled libffi for windows users, and libffi uses libtool.  If
we use cross-compiler version of gcc instead of its ld conterpart, we
fail to compile fiddle.  That should not be what we want.

Caused CI failures on environments without sane C++ compilers.

Historically `$program_transform_name` has been put in front of
`AC_CANONICAL_TARGET`.  Previous commit changes it, which affects the
name of generated ruby binary when cross-compiling.  I _guess_ the
historical behaviour is a bug (name of ruby binary shall honour --target
configure option I think), but anyways here I preserve that questionable
way.

These days as link-time optimisations spread accross compilers, they
tend to ship their own version of ld, ar, etc.  Why not detect such
things if any.  Users can select compilers by ./configure --with-gcc=
whatever, or select individual tool by e.g. ./configure NM=whatever.

The added AC_ARG_VAR macros enrich ./configure --help output.

Because we check HAVE_STMT_AND_DECL_IN_EXPR in configure, it is peoven
to work in C.  But C++ situation can be different.  Oracle Developer
Studio is another example of such things.

I found a way to suppress particular kind of warnings via pragmas.

Oracle Developer Studio's C++ preprocessor started to understand
__has_cpp_attribute since version 12.5, and is broken.  After looking
around for a while I found Boost and ICU also had this issue before.
Let me add workaround for it.

Removes references to *-convertible thingies.

Removes references to *-convertible thingies.

Removes references to *-convertible thingies.

If the passed string is frozen, bare and not shared, then there
is no need to duplicate it.

Ref: 4ab69ebb
Ref: https://bugs.ruby-lang.org/issues/11386

https://github.com/ruby/racc/commit/a1aeecff03

Because Editline erases all histories in Readline::HISTORY when
Readline.readline is called.

It seems almost no internal codes use RHASH_TBL any longer.  Why not
just eliminate it entirely, so that the macro can be purely ext-only.

Former ROBJECT_IV_INDEX_TBL macro included RCLASS_IV_INDEX_TBL, which is
not disclosed to extension libraies.  The macro was kind of broken.  Why
not just deprecate it, and convert the internal use into an inline
function.

Compaction makes collisions in the weak map more likely.  For now, clear
the tracing information before the test run so we get accurate results

The allocation tracing code keeps essentially a weak reference to
objects that have been allocated (storing the allocation information
along with the weak ref).  Compacting the heap would break references in
this weak map, so the wrong values could be returned.

This commit just updates the values in the weak ref in order to fix the
allocation tracing book keeping

```
/home/chkbuild/chkbuild/tmp/build/20200818T123003Z/ruby/test/irb/test_history.rb:16:
warning: assigned but unused variable - result_output
/home/chkbuild/chkbuild/tmp/build/20200818T123003Z/ruby/test/irb/test_history.rb:34:
warning: assigned but unused variable - result_output
/home/chkbuild/chkbuild/tmp/build/20200818T123003Z/ruby/test/irb/test_history.rb:57:
warning: assigned but unused variable - result_output
/home/chkbuild/chkbuild/tmp/build/20200818T123003Z/ruby/test/irb/test_history.rb:80:
warning: assigned but unused variable - result_output
```