sdk: preserve pthread compiler semantics

[brandonpayton]

Why

AX_PTHREAD is a standard Autoconf configure check that discovers how a compiler enables POSIX threads. The SDK is Kandelo's software development kit, which drives Clang for package builds.

XZ's standard AX_PTHREAD configure probe compiles with -pthread and requires Clang's _REENTRANT definition. Kandelo's SDK silently removed -pthread, so the probe failed even though Kandelo provides working pthread APIs. That left PTHREAD_CFLAGS and PTHREAD_LIBS empty and produced incomplete liblzma.pc metadata.

This is an SDK compiler-driver defect exposed by the Homebrew XZ build. The fix belongs in the platform, not in an XZ Formula cache override.

What changed

• forward -pthread to Clang in the TypeScript SDK driver;
• preserve the same flag in the packaged in-guest shell driver;
• keep -lpthread as the existing compatibility no-op because pthread symbols come from musl's libc.a;
• document the compiler semantics; and
• add unit and Autoconf-shaped compile/link regressions that require _REENTRANT and real pthread symbols.

The change preserves #913's exact linker-input ordering: -pthread enters the single ordered forwarding stream at the position supplied by the caller.

Ordering

This PR is stacked on #913 (sdk: preserve static linker argument order) because both changes touch the SDK argument-forwarding path. Merge #913 first, then retarget this PR to main.

ABI

No ABI bump is required. This changes how the compiler driver treats an existing source/build flag; it does not change syscalls, marshalling, channel or memory layouts, exported runtime interfaces, libc signatures, fork frames, or ABI metadata. The canonical ABI snapshot/version check is unchanged and passes.

Consumer proof

A disposable future-platform integration combined exact #936 + #913 + #874 + #883 with this fix (synthetic tree f1614ed97525db9d9f80bac987645787ecdecad5). The supported XZ Homebrew build then reported:

• AX_PTHREAD selected PTHREAD_CFLAGS=-pthread and PTHREAD_LIBS=-lpthread;
• installed liblzma.pc recorded Libs.private: -pthread -lpthread;
• the complete XZ source build, both final-artifact validators, literal Formula test, bottle inspection, handoff creation, and independent handoff validation passed.

This evidence is local and unpublished; no XZ-specific source or Formula workaround is part of this PR.

Validation

Exact head: a27676e3b397996417397c0428a46ea12bf54ecc
Exact tree: ce3ea9b1b287919eb46216bd735fa49b6f3a66ac

• focused SDK flag/compiler tests: 41/41 passed;
• sdk: preserve static linker argument order #913 linker-order plus Autoconf pthread integration tests: 2/2 passed;
• full SDK suite: 66/66 passed;
• both the TypeScript and packaged compiler drivers emit #define _REENTRANT 1 for -pthread;
• ./scripts/dev-shell.sh bash scripts/check-abi-version.sh: passed;
• packaged driver shell syntax and git diff --check: passed.

All validation ran through scripts/dev-shell.sh. No XZ-specific patch is included.

[github-actions]

Phase B-1 matrix build status — pr-938-staging

ABI v39. 71 built, 0 failed, 71 total.

Package	Arch	Status	Sha
icu	wasm32	built	f2e695e4
libcurl	wasm32	built	2d9fc0e9
libcxx	wasm32	built	aca0859f
libcxx	wasm64	built	d3353c8b
libiconv	wasm32	built	c8ee6514
libpng	wasm32	built	d23eb9a8
libxml2	wasm32	built	a2c268e1
libzip	wasm32	built	a8a2053d
openssl	wasm32	built	7ec09ece
openssl	wasm64	built	5138eb1d
sqlite	wasm32	built	9964d42c
sqlite	wasm64	built	3f6d4e53
zlib	wasm32	built	6e32642b
zlib	wasm64	built	7cb3e2c3
bc	wasm32	built	b5e1e72e
bzip2	wasm32	built	8360bb15
coreutils	wasm32	built	9736f5c1
curl	wasm32	built	08f31048
dash	wasm32	built	0dda8fe7
diffutils	wasm32	built	a65dc325
dinit	wasm32	built	021a7147
fbdoom	wasm32	built	da6efb4e
file	wasm32	built	d26064da
findutils	wasm32	built	2b10b7a8
gawk	wasm32	built	eaf1ee46
git	wasm32	built	46e089de
grep	wasm32	built	89d985ed
gzip	wasm32	built	b473d3cb
hello	wasm32	built	1b5808b4
kandelo-sdk	wasm32	built	9d7c338f
kernel	wasm32	built	1cd93cb0
less	wasm32	built	ecc93e5f
lsof	wasm32	built	3d9ef32f
m4	wasm32	built	d0f4764c
make	wasm32	built	c14f3ec3
mariadb	wasm32	built	af817f57
mariadb	wasm64	built	2ce5a72e
modeset	wasm32	built	1b6f3166
msmtpd	wasm32	built	c299471b
nano	wasm32	built	59e63305
ncurses	wasm32	built	2666604b
netcat	wasm32	built	edaa94f1
nginx	wasm32	built	5b281460
php	wasm32	built	e33d9621
posix-utils-lite	wasm32	built	124efa0c
ruby	wasm32	built	b70c442b
sed	wasm32	built	e30232c4
spidermonkey	wasm32	built	5439f082
tar	wasm32	built	0db63c2b
tcl	wasm32	built	fef4bbf1
unzip	wasm32	built	71efacd8
userspace	wasm32	built	6dead609
vim	wasm32	built	8227dba0
wget	wasm32	built	bc3dfdbd
xz	wasm32	built	6375c5a6
zip	wasm32	built	0f9d71ae
zstd	wasm32	built	b233bb34
bash	wasm32	built	d06904ca
mariadb-test	wasm32	built	e1d1bc49
mariadb-vfs	wasm32	built	36e04d39
mariadb-vfs	wasm64	built	c6ae7efa
nethack	wasm32	built	286f0ea1
node	wasm32	built	092ac57a
spidermonkey-node	wasm32	built	113b2b34
vim-browser-bundle	wasm32	built	1c521853
nethack-browser-bundle	wasm32	built	73ba24a7
rootfs	wasm32	built	f90ec0a3
shell	wasm32	built	0e21ee6d
lamp	wasm32	built	eb5bbda0
node-vfs	wasm32	built	73f29b3c
wordpress	wasm32	built	39952083

_{Auto-generated; replaced on each push. Raw data in the publish-status workflow artifact.}

[brandonpayton]

Absorbed by rebase-merged #967. Main is now 6efb411 with the sealed tree b133bfa018f7e2bc84c4845edf2f945b2f844954, and ABI 41 activation completed successfully in run 29550069085. The absorption audit confirmed this PR\x27s behavior and tests are present in that merged tree, so this source PR is closed and its branch is deleted.