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jit: don't hash the mutable CFLAGS into the JIT module name (#895)

_compilation_signature hashed sysconfig's CFLAGS build variable. On macOS
setuptools rewrites that variable in place on every compile, in
_osx_support._override_all_archs, and the rewrite is not idempotent: its
regex requires trailing whitespace, so the "-arch" it appends at the end
of the string cannot be stripped next time. A second copy is appended, and
from then on each compile strips the first and appends another, growing the
string by one space every time.

Every compile therefore changed the value the next call hashed, so every
form got a fresh module name. Neither the on-disk cache nor the in-process
memo could be hit, and each form was regenerated and recompiled from
scratch. In a DOLFINx test calling form() 140 times on one form this meant
140 code generations and 140 compiler invocations, 208s against 1.4s, and
560 files left in the cache directory.

Use EXT_SUFFIX and get_platform() instead. Between them they cover
everything CFLAGS and SOABI did:

- EXT_SUFFIX gives implementation, Python version and ABI flags, and is
  defined on every platform, so the win32 branch is no longer needed.
- get_platform() gives the architecture, which EXT_SUFFIX omits on macOS,
  and the deployment target.

Neither is in _osx_support._UNIVERSAL_CONFIG_VARS, so neither is rewritten.
Dropping the SDK path and optimisation flags that CFLAGS carried also stops
an Xcode or SDK upgrade needlessly invalidating the whole cache.


Claude-Session: https://claude.ai/code/session_01E9KDSv5nDT5D7V83P1NKtY

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

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