[cgal] Make GMP/MPFR an optional default feature

[acgetchell]

Is your feature request related to a problem? Please describe.

The cgal port currently depends unconditionally on gmp and mpfr.
CGAL itself supports a Boost.Multiprecision exact-number backend, so consumers
that only need CGAL's exact predicates cannot currently remove these two native
dependencies through manifest features.

CGAL documents Boost.Multiprecision as an alternative exact-number backend:
https://doc.cgal.org/latest/Manual/thirdparty.html#thirdpartyBoostMp

Proposed solution

Make GMP/MPFR an optional default feature of the cgal port:

• add a feature such as gmp;
• include that feature in default-features, preserving the port's current
  behavior;
• move the gmp and mpfr dependencies into that feature; and
• when the feature is disabled, configure CGAL with:
  • CGAL_DISABLE_GMP=ON
  • CGAL_CMAKE_EXACT_NT_BACKEND=BOOST_BACKEND

A Boost-only manifest consumer could then use:

{
  "dependencies": [
    {
      "name": "cgal",
      "default-features": false
    }
  ]
}

The existing default would continue to install and select GMP/MPFR, so this
should be backward compatible for current consumers.

Validation

I tested this port shape against vcpkg baseline
9e593bb18ea69cc5095e012465dcd675a822ed0d with CGAL 6.2 and Boost 1.91
on arm64 macOS.

The no-default-feature configuration:

• installed CGAL without gmp or mpfr in the dependency graph;
• compiled with CGAL_DISABLE_GMP and without CGAL_USE_GMP /
  CGAL_USE_MPFR;
• passed an EPICK probe covering an almost-coplanar orientation and
  co-spherical 3D Delaunay insertion;
• linked without GMP or MPFR dynamic libraries; and
• passed a downstream project's 127-test suite and produced byte-identical
  canonical reference fixtures versus the default CGAL backend.

This validation is currently limited to arm64 macOS; the normal vcpkg CI
matrix would still be needed for the port change.

I would be happy to prepare a PR if maintainers agree with the feature shape
(including the preferred feature name).