sagemathgh-36599: sage-env: identify the version of command-line tool…

…s (OS X) and set LDFLAGS accordingly

    
Identify the version of command-line tools in use (OS X), and use the
version to set LDFLAGS.

<!-- ^^^^^
Please provide a concise, informative and self-explanatory title.
Don't put issue numbers in there, do this in the PR body below.
For example, instead of "Fixes sagemath#1234" use "Introduce new method to
calculate 1+1"
-->
<!-- Describe your changes here in detail -->

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 sagemath#36337 (doctest failures)
- Fixes sagemath#36342 (scipy building,
replacing sagemath#36523).

<!-- Why is this change required? What problem does it solve? -->
<!-- If this PR resolves an open issue, please link to it here. For
example "Fixes sagemath#12345". -->
<!-- If your change requires a documentation PR, please link it
appropriately. -->

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->
<!-- If your change requires a documentation PR, please link it
appropriately -->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
<!-- Feel free to remove irrelevant items. -->

- [X] The title is concise, informative, and self-explanatory.
- [X] The description explains in detail what this PR is about.
- [X] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on
- sagemath#12345: short description why this is a dependency
- sagemath#34567: ...
-->

<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
    
URL: sagemath#36599
Reported by: John H. Palmieri
Reviewer(s): Matthias Köppe

src/bin/sage-env

@@ -374,7 +374,14 @@ if [ -n "$PYTHONHOME" ]; then
 fi
 
 if [ -n "$SAGE_LOCAL" ]; then
-    LDFLAGS="-L$SAGE_LOCAL/lib -Wl,-rpath,$SAGE_LOCAL/lib $LDFLAGS"
+    # On OS X, test whether "ld-classic" is present in the installed
+    # version of the command-line tools. If so, we add "-ld_classic"
+    # to LD_FLAGS. See #36599.
+    if [ "$UNAME" = "Darwin" ] && [ -x "$(xcode-select -p)/usr/bin/ld-classic" ] ; then
+        LDFLAGS="-L$SAGE_LOCAL/lib -Wl,-ld_classic,-rpath,$SAGE_LOCAL/lib $LDFLAGS"
+    else
+        LDFLAGS="-L$SAGE_LOCAL/lib -Wl,-rpath,$SAGE_LOCAL/lib $LDFLAGS"
+    fi
     if [ "$UNAME" = "Linux" ]; then
         LDFLAGS="-Wl,-rpath-link,$SAGE_LOCAL/lib $LDFLAGS"
     fi