build(cgal): use CGAL_DISABLE_GMP, drop the GMP dependency

[krystophny]

Summary

Enabling SIMPLE_ENABLE_CGAL pulled in GMP and broke the build on machines
without GMP development headers (most HPC nodes ship libgmp.so but no
gmp.h). CGAL 6.0.1's bundled CORE defines CORE::BigInt as
boost::multiprecision::mpz_int, which requires gmp.h. The previous CMake
tried the system GMP, then fell back to building GMP 6.3.0 from source via an
ExternalProject. Both paths are fragile, and the CORE + GMP + Boost.Multiprecision
combination fails to compile regardless (CORE::BigInt does not name a type,
mpz_int in namespace boost::multiprecision does not name a type), which is
what kills the gfortran build whenever a stale cache leaves SIMPLE_ENABLE_CGAL=ON.

CGAL_DISABLE_GMP switches CORE to boost::multiprecision::cpp_int, which is
header-only and always available with Boost. The wall feature then needs only
Boost headers, no GMP at all.

• Define CGAL_DISABLE_GMP on the stl_wall_cgal target.
• Remove the system-GMP probe and the GMP-from-source ExternalProject.

Verification

Before, compiling the CGAL wall TU without gmp.h on the include path fails:

$ g++ -std=c++17 -DSIMPLE_ENABLE_CGAL -I<cgal>/include -c src/wall/stl_wall_cgal.cpp
CGAL/CORE_arithmetic_kernel.h:44:19: error: 'BigInt' in namespace 'CORE' does not name a type
... (771 errors, CORE needs GMP)

After (this change adds -DCGAL_DISABLE_GMP):

$ g++ -std=c++17 -DSIMPLE_ENABLE_CGAL -DCGAL_DISABLE_GMP -I<cgal>/include -c src/wall/stl_wall_cgal.cpp
exit 0   (0 errors)

g++ 12.2, CGAL 6.0.1, no GMP on the system. The wall feature builds with Boost
headers only.