Support developer builds of CppInterOp/LLVM, add project verification and robust fallbacks

[aaronj0]

Developing CppInterOp or LLVM against cppjit requires building from local trees instead of the pinned clone. CPPINTEROP_SOURCE_DIR now builds CppInterOp from a local checkout with incremental rebuilds, and a post-config check rejects source trees that are not CppInterOp. Validated against a fetch of ci-workflows LLVM 22 and an uninstalled LLVM 21.1.8 build tree. This change also improves how robust the build system is with various permutations of developer setups (custom CppInterOp/LLVM development)

The behavior of cppjit's build system with this change is:

pip install . --config-settings=cmake.define.<...>	CppInterOp	LLVM
none (basic install)	clones and builds the pinned CPPINTEROP_GIT_TAG	find_package(LLVM): $CONDA_PREFIX hint, then the system search
LLVM_DIR=<dir>	clones and builds the pin	uses provided install prefix or build tree only, no fallback as this is a developer override
CPPINTEROP_GIT_REPOSITORY=<url> / CPPINTEROP_GIT_TAG=<ref>	clones the given remote/ref and builds it	per the rows above
CPPINTEROP_SOURCE_DIR=<dir>	builds the local tree in place with incremental rebuilds (output is streamed)	per the rows above

Priority:

• CPPINTEROP_SOURCE_DIR over CPPINTEROP_GIT_REPOSITORY/CPPINTEROP_GIT_TAG
• LLVM_DIR over the $CONDA_PREFIX hint over the system search.

The two axes combine freely, and the selected LLVM is forwarded to the CppInterOp sub-build.

The negative cases each fail at config time with actionable feedback:

Case	Result
no LLVM found	No LLVM CMake package found. Install LLVM 20-22 development packages (apt: llvm-22-dev libclang-22-dev; conda: llvmdev clangdev), or point cppjit at your own LLVM build with -DLLVM_DIR=...
LLVM_DIR invalid	No LLVMConfig.cmake under LLVM_DIR (<path>); expected <install prefix or build tree>/lib/cmake/llvm
Clang_DIR invalid	No ClangConfig.cmake under Clang_DIR (<path>)
LLVM outside 20-22	LLVM <version> is unsupported: the currently supported CppInterOp version (CPPINTEROP_GIT_TAG) only supports LLVM 20-22
CPPINTEROP_SOURCE_DIR is not a CMake tree	the sub-configure reports the missing CMakeLists.txt
the source or ref is not CppInterOp	pre-build, post scikit-build-core config: The provided source path override declares project '<name>', not CppInterOp

The supported range is defined once next to the pin (CPPJIT_LLVM_VERSION_MIN/MAX, updated together with the supported CppInterOp version), and the Clang lookup is now confined to the discovered LLVM's own package dir, preventing a mismatched system clang from breaking the config on a machine with several toolchains installed.

[github-actions]

Test Results

Configuration	Result
macos-26-intel-llvm21-py3.14-cxx20	= 438 passed, 44 skipped, 85 xfailed, 32 xpassed, 100 warnings in 109.62s (0:01:49) =
macos-26-llvm21-py3.14-cxx20	==== 439 passed, 37 skipped, 93 xfailed, 30 xpassed, 100 warnings in 45.14s ====
ubuntu-24.04-llvm21-py3.14-cxx20-vg	====== 554 passed, 25 skipped, 18 xfailed, 2 xpassed in 96.31s (0:01:36) =======
ubuntu-24.04-llvm22-py3.14-cxx20	====== 554 passed, 25 skipped, 19 xfailed, 1 xpassed in 117.43s (0:01:57) ======
ubuntu-24.04-llvm22-py3.14-cxx20-cling	====== 544 passed, 25 skipped, 27 xfailed, 3 xpassed in 87.92s (0:01:27) =======

[guitargeek]

LGTM! Consider CMake 3.20 compatibility if possible, but if it can't be done so be it. LLVM will also require CMake 3.31, starting from LLVM 24.