devel/root: Disable runtime_cxxmodules to avoid modulemap issues

A recent change in LLVM/Clang modulemap handling has caused ROOT (which builds
its own patched version of LLVM/Clang for JIT C/C++ compilation, e.g. from the
ROOT terminal) in the upstream source code to hack modulemap definitions on a
case-by-case OS release-dependent basis on macOS, which also compiles ROOT with
a LLVM/Clang compiler.
Once a more robust solution has been found in the upstream code, we can revert
to enabling 'runtime_cxxmodules' in the port.
For now, my attempts at continually patching ROOT for FreeBSD 13.3+, 14.0+ and
15.0 have proven too error-prone, and a package built on 14.0, for example, and
distributed to both 14.0 and 14.1 would otherwise only work on 14.0 in the best
of cases.

PR: 282756

(cherry picked from commit 867e49b82ad9850646f1c8114317ef50d38a1d2b)