libxml2: preserve resolver zlib inputs in manual build

[brandonpayton]

Purpose

Keep libxml2's manual source build on the package resolver's declared zlib dependency path.

The build script runs configure with --with-zlib, then bypasses the generated Makefiles and compiles the libxml2 sources itself. That manual compile omitted the resolver zlib include directory, so CI failed at tree.c: fatal error: zlib.h: No such file or directory even though the resolver had supplied a complete zlib prefix.

Change

• require both the resolved zlib archive and header
• pass the resolved zlib include/library paths through configure
• preserve the resolved zlib include path in the manual compile flags
• use an argument array for the manual compiler invocation

This changes package build behavior only. It does not change the Kandelo ABI.

Validation

All commands ran through scripts/dev-shell.sh.

• bash -n packages/registry/libxml2/build-libxml2.sh
• full wasm32 libxml2 source build from this branch against the current main SDK: produced lib/libxml2.a
• full wasm32 libxml2 source build from this branch against the SDK ordering fix in SDK: preserve user linker order after platform glue #899: configure found both zlib.h and gzread in -lz, then produced lib/libxml2.a
• linked a smoke module using both xmlReadMemory and zlibVersion against resolver-produced libxml2 and zlib prefixes
• wasm-validate --enable-threads --enable-exceptions --enable-function-references on the smoke module
• confirmed ABI export value 18 and no unresolved libxml2/zlib imports in the smoke module
• git diff --check

shellcheck was not run because it is not available in the declared dev shell.

[brandonpayton]

Exact-head CI classification for 67bfdb225:

• Browser, cargo-kernel, fork-instrument, libc, POSIX, and Sortix suites passed.
• Full Vitest ran 125 files / 898 tests; 116 files and 869 tests passed, with the sole failure at host/test/teardown-reclaim.test.ts:71.
• That assertion is the known fixed-delay teardown readiness race addressed by open PR Make teardown reclamation CI wait for a blocked worker #889; it is unrelated to this PR's libxml2 resolver zlib include/library propagation.

Failing job: https://github.com/Automattic/kandelo/actions/runs/29195911562/job/86660143205

No broader green claim is made while the required check remains red.

[brandonpayton]

Merged PR #907 supersedes this work through the holistic libxml2 resolver-input correction now on main at 4a449540f. The merged implementation is strengthened rather than a verbatim merge of this head.