gh-36599: sage-env: identify the version of command-line tools (OS X) and set LDFLAGS accordingly
Identify the version of command-line tools in use (OS X), and use the
version to set LDFLAGS.
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In sage-env, save the version of command-line tools being used in
XCLT_VERSION. If it's too recent, we set LDFLAGS to solve some problems:
- Fixes #36337 (doctest failures)
- Fixes https://github.com/sagemath/sage/issues/36342 (scipy building,
replacing #36523).
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URL: https://github.com/sagemath/sage/pull/36599
Reported by: John H. Palmieri
Reviewer(s): Matthias Köppe
