Update README for current toolchains and dead upstream links

[tas50]

Description

The README had drifted a long way from reality, including several links that no longer resolve. Every link in the file was checked before editing.

Dead or moved links

Old	Status	Replacement
gecode.org	Offline — apex returns 521, www is NXDOMAIN	github.com/Gecode/gecode
downloads.chef.io/chef-dk/windows/	404, ChefDK is EOL	Removed
rubyinstaller.org/add-ons/devkit/	404, standalone DevKit retired	RubyInstaller2 + ridk install
opscode/dep-selector, opscode/omnibus-chef	Org renamed	chef/* equivalents

Requirements that were never documented

• ext/libgecode3/extconf.rb shells out to libtoolize, aclocal, autoheader, automake, autoconf and autoupdate on non-Windows platforms, so autotools are a hard requirement and not just a C++ compiler. The README previously said nothing about this. Adds a per-platform install table.
• Homebrew installs GNU libtool as glibtoolize to avoid colliding with Apple's unrelated libtool, but extconf.rb invokes bare libtoolize. A stock brew install libtool therefore still fails on macOS. Documents the libexec/gnubin PATH fix.
• USE_SYSTEM_GECODE is close to unusable in practice now. Debian and Ubuntu ship Gecode 6.2.0 and Homebrew ships 6.4.0, while dep-selector needs 3.x, so the option is only realistic for someone who has built 3.7.3 themselves or is packaging inside a larger build such as Omnibus. Reframed accordingly rather than left as a generic "make sure it's version 3" warning.

Corrections

• The concurrency claim said make -j was capped at 4. extconf.rb computes min(cores, 4) + 1, so it is up to five jobs.
• The rake native example still referenced 1.3.0; the current version is 1.3.5.

Additions

• A short "Project status" section explaining that being pinned to a 2012 release of Gecode is deliberate rather than neglect, and noting that the resulting wall of compiler warnings on modern GCC is expected.
• A "Development" section covering how to build the gem and how to exercise the vendored Gecode compile locally.
• CI and gem version badges.

Note that the Development section links to .github/workflows/ci.yml, which is added in a follow-up PR. That link will 404 until the CI PR lands.

Issues Resolved

None filed; this is documentation drift found while reading through the repo.

Check List

• New functionality includes tests
• All tests pass
• All commits have been signed-off for the Developer Certificate of Origin.

Documentation-only change, so the test boxes do not apply. No code is touched.