cpython ./configure's -pthread detection is misbehaving on some variants

I am getting CI failures on #592 on some variants because of missing pthread symbols because of a missing `-pthread` on the compiler command line. Compare, for instance, [x86_64_v2-unknown-linux-gnu / 3.9 / debug](https://github.com/astral-sh/python-build-standalone/actions/runs/14718002189/job/41306023985?pr=592) which fails with

```
2025-04-28T21:36:18.8638916Z cpython-3.9> clang -L/tools/deps/lib -Wl,--exclude-libs,ALL -Xlinker -export-dynamic -o Programs/_testembed Programs/_testembed.o -L. -lpython3.9d -lpthread -ldl -lutil -lm -lbz2 -lffi -ldl -lm -lncursesw -lpanelw -lncursesw -ldb -lmpdec -lexpat -lcrypto -l:libatomic.a -llzma -lrt -lsqlite3 -lssl -lcrypto -l:libatomic.a -ltcl8.6 -ltk8.6 -lX11 -lxcb -lXau -luuid -lm -lm -lexpat -ledit -lncursesw -lz -lm
2025-04-28T21:36:19.1392376Z cpython-3.9> /tools/host/bin/ld: /tools/deps/lib/libtcl8.6.a(tclUnixNotfy.o): in function `Tcl_InitNotifier
2025-04-28T21:36:19.1394260Z cpython-3.9> ':
2025-04-28T21:36:19.1395858Z cpython-3.9> tclUnixNotfy.c:(.text+0x7d): undefined reference to `pthread_atfork'
2025-04-28T21:36:19.1910841Z cpython-3.9> clang: error: linker command failed with exit code 1 (use -v to see invocation)
```

versus [x86_64-unknown-linux-gnu / 3.9 / debug](https://github.com/astral-sh/python-build-standalone/actions/runs/14718002189/job/41306015334?pr=592) which succeeds with the corresponding build line being

```
2025-04-28T21:37:46.4684015Z cpython-3.9> clang -pthread -L/tools/deps/lib -Wl,--exclude-libs,ALL -Xlinker -export-dynamic -o Programs/_testembed Programs/_testembed.o -L. -lpython3.9d -lpthread -ldl -lutil -lm -lbz2 -lffi -ldl -lm -lncursesw -lpanelw -lncursesw -ldb -lmpdec -lexpat -lcrypto -l:libatomic.a -llzma -lrt -lsqlite3 -lssl -lcrypto -l:libatomic.a -ltcl8.6 -ltk8.6 -lX11 -lxcb -lXau -luuid -lm -lm -lexpat -ledit -lncursesw -lz -lm
```

If you dig a bit earlier in the build logs, the x86_64_v2 (unsuccessful) build does

```
2025-04-28T21:35:49.0615195Z cpython-3.9> checking whether pthreads are available without options...
2025-04-28T21:35:49.0616630Z cpython-3.9> no
2025-04-28T21:35:49.0618854Z cpython-3.9> checking whether clang accepts -Kpthread...
2025-04-28T21:35:49.0619625Z cpython-3.9> no
2025-04-28T21:35:49.0622574Z cpython-3.9> checking whether clang accepts -Kthread... no
2025-04-28T21:35:49.0622969Z cpython-3.9> checking whether clang accepts -pthread...
2025-04-28T21:35:49.0623276Z cpython-3.9> no
2025-04-28T21:35:49.0623591Z cpython-3.9> checking whether clang++ also accepts flags for thread support...
2025-04-28T21:35:49.0623927Z cpython-3.9> no
```

while the x86_64 (successful) build does

```
2025-04-28T21:37:20.2180518Z cpython-3.9> checking whether pthreads are available without options...
2025-04-28T21:37:20.2551345Z cpython-3.9> no
2025-04-28T21:37:20.2552261Z cpython-3.9> checking whether clang accepts -Kpthread...
2025-04-28T21:37:20.2676725Z cpython-3.9> no
2025-04-28T21:37:20.2677100Z cpython-3.9> checking whether clang accepts -Kthread...
2025-04-28T21:37:20.2800777Z cpython-3.9> no
2025-04-28T21:37:20.2804876Z cpython-3.9> checking whether clang accepts -pthread...
2025-04-28T21:37:20.3187186Z cpython-3.9> yes
2025-04-28T21:37:20.3187607Z cpython-3.9> checking whether clang++ also accepts flags for thread support...
2025-04-28T21:37:20.3793651Z cpython-3.9> yes
```

This does not appear to have been introduced by my PR; the same ./configure discrepancy happens on HEAD (compare [this run](https://github.com/astral-sh/python-build-standalone/actions/runs/14361390782/job/40264100181) and [this run](https://github.com/astral-sh/python-build-standalone/actions/runs/14361390782/job/40264036290), for instance), though the fact that ./configure decides not to pass `-pthread` does not fail the build on HEAD. I do not know if it has any unwanted side effects.

Almost certainly what is going on is that the test program is failing to run for some other reason, and ./configure is getting confused.

We need to resolve this as it blocks #592, and I'm a little bit uncomfortable with the fact that this discrepancy is happening and I'd like to think a bit about noticing and detecting discrepancies like this in the general case.