Fix CMake installs for C++-only builds

[jeongseok-meta]

Motivation

C++-only and header-only builds should install a conventional CMake package:

• With every JIT backend disabled, install rules must not reference JIT targets that were never created.
• With JIT enabled and Python disabled, libraries belong in the standard CMake runtime and library directories rather than the Python package directory.

Installing public drjit-core headers without JIT is intentionally handled separately in #527. This PR corresponds to conda-forge's 0002-Fix-install-destination-paths.patch. Once this change is included in an upstream release, the feedstock can remove that patch.

Changes

• Guard drjit-core, drjit-extra, and nanothread target installation with DRJIT_ENABLE_JIT.
• Use CMAKE_INSTALL_BINDIR and CMAKE_INSTALL_LIBDIR when Python is disabled.
• Preserve the existing drjit/ library layout when Python is enabled.

This PR does not move public headers or change Python stub generation or installation.

Testing

• git diff --check
• No-JIT/no-Python configure, build, and install passed.
• LLVM 21 C++ configure, build, and install placed drjit-core, drjit-extra, and nanothread under lib/, not drjit/.
• An external find_package(drjit) consumer built successfully; the installed-header smoke test and live LLVM test both passed.
• Python 3.13 + LLVM 21 configure, build, and install; drjit and drjit.auto imported, a live LLVM calculation passed, all generated stubs were installed, and libraries remained under drjit/.

[wjakob]

Hi @jeongseok-meta,

I don't think it would be a good idea to release "inferior" packages without stubs that are produced by cross-compiling. The stub generation process fundamentally requires the ability to build on the platform that can import the compiled wheels.

I think that fixing this by building on a wider set of platforms will address most of the issues you are pursuing here.

[jeongseok-meta]

Thanks for the feedback. I moved the stub issue out of this PR. conda-forge/drjit-cpp-feedstock#49 now builds macOS ARM packages natively, and CI confirmed that the Python 3.10–3.13 packages include the generated stubs.

I've narrowed this PR to the C++ install fixes: handling no-JIT builds, using standard library destinations when Python is disabled, preserving the existing Python package layout, and installing the required drjit-core headers.

I tested no-JIT and LLVM C++ installs with external find_package(drjit) consumers, plus a Python build to confirm that imports, stubs, and the existing library layout remain unchanged.