Win: build in shallow prefix to avoid long path failures

pyproject.toml

@@ -106,4 +106,9 @@ test-command = "pytest --import-mode=importlib {project}/tests/test_castxml.py"
 # Build with Ninja instead of Visual Studio/MSBuild: much faster for LLVM, and
 # the superbuild switches its LLVM/Clang/CastXML builds to Ninja whenever it
 # finds ninja anyway. The workflow sets up the MSVC environment Ninja needs.
-environment = { CMAKE_GENERATOR = "Ninja" }
+#
+# Build in a short directory at the drive root. The LLVM build nests deeply
+# (FetchContent, then the superbuild's ExternalProjects), and under the default
+# %TEMP%\cibw-sdist-*\castxml-<version>\build\<wheel tag> its paths exceed what
+# the MSVC tools can open (MAX_PATH), even with Windows long paths enabled.
+environment = { CMAKE_GENERATOR = "Ninja", SKBUILD_BUILD_DIR = "C:/cxb" }